diff --git a/src/bdew_datetimes/__init__.py b/src/bdew_datetimes/__init__.py
index e1d038c..516b4dc 100644
--- a/src/bdew_datetimes/__init__.py
+++ b/src/bdew_datetimes/__init__.py
@@ -7,3 +7,4 @@
 from .calendar import create_bdew_calendar
 
 GERMAN_TIME_ZONE = timezone("Europe/Berlin")
+__all__ = ["GERMAN_TIME_ZONE", "create_bdew_calendar"]
diff --git a/src/bdew_datetimes/calendar.py b/src/bdew_datetimes/calendar.py
index b3acd58..e0229d2 100644
--- a/src/bdew_datetimes/calendar.py
+++ b/src/bdew_datetimes/calendar.py
@@ -4,9 +4,10 @@
 """
 
 from datetime import date
+from typing import Any
 
 from holidays import HolidayBase, HolidaySum
-from holidays.constants import APR, DEC
+from holidays.constants import APR, DEC  # type:ignore[attr-defined]
 from holidays.countries.germany import Germany
 
 
@@ -21,10 +22,10 @@ class BdewDefinedHolidays(HolidayBase):
 
     """
 
-    def __init__(self, observed: bool = False, **kwargs):
+    def __init__(self, observed: bool = False, **kwargs: Any) -> None:
         super().__init__(observed=observed, **kwargs)
 
-    def _populate(self, year):
+    def _populate(self, year: int) -> None:
         self[date(year, DEC, 24)] = "Heiligabend"
         self[date(year, DEC, 31)] = "Silvester"
         if year == 2025:
diff --git a/src/bdew_datetimes/periods.py b/src/bdew_datetimes/periods.py
index cdcc351..94013b8 100644
--- a/src/bdew_datetimes/periods.py
+++ b/src/bdew_datetimes/periods.py
@@ -11,7 +11,7 @@
 from typing import Literal, Optional, Union
 
 from dateutil.relativedelta import relativedelta
-from holidays import SAT, SUN
+from holidays import SAT, SUN  # type:ignore[attr-defined]
 
 from bdew_datetimes import GERMAN_TIME_ZONE, create_bdew_calendar
 
diff --git a/tests/test_calendar.py b/tests/test_calendar.py
index 9b7d8c4..4f55d1c 100644
--- a/tests/test_calendar.py
+++ b/tests/test_calendar.py
@@ -1,6 +1,6 @@
 from datetime import date, datetime
 
-import pytest  # type:ignore[import]
+import pytest
 from holidays import DateLike
 
 from bdew_datetimes.calendar import BdewDefinedHolidays, create_bdew_calendar
@@ -15,7 +15,7 @@
         pytest.param(date(2070, 12, 24), id="arbitrary Heiligabend"),
     ],
 )
-def test_bdew_holidays(expected_holiday: date):
+def test_bdew_holidays(expected_holiday: date) -> None:
     calendar = BdewDefinedHolidays()
     assert expected_holiday in calendar
 
@@ -33,7 +33,9 @@ def test_bdew_holidays(expected_holiday: date):
         pytest.param(date(2025, 4, 4), True, id="Sonderfeiertag 2025 (BDEW)"),
     ],
 )
-def test_create_bdew_calendar(test_date: date, expected_is_in_calendar: bool):
+def test_create_bdew_calendar(
+    test_date: date, expected_is_in_calendar: bool
+) -> None:
     calendar = create_bdew_calendar()
     if expected_is_in_calendar:
         assert test_date in calendar
@@ -41,7 +43,7 @@ def test_create_bdew_calendar(test_date: date, expected_is_in_calendar: bool):
         assert test_date not in calendar
 
 
-def test_holiday_calendar_obj():
+def test_holiday_calendar_obj() -> None:
     calendar = create_bdew_calendar()
 
     assert not calendar.observed
@@ -62,6 +64,6 @@ def test_holiday_calendar_obj():
         pytest.param(int(datetime(2022, 1, 1, 22, 16, 59).timestamp())),
     ],
 )
-def test_holiday_in_calendar(candidate: DateLike):
+def test_holiday_in_calendar(candidate: DateLike) -> None:
     calendar = create_bdew_calendar()
     assert candidate in calendar
diff --git a/tests/test_german_strom_and_gas_tag.py b/tests/test_german_strom_and_gas_tag.py
index 18af61c..375297c 100644
--- a/tests/test_german_strom_and_gas_tag.py
+++ b/tests/test_german_strom_and_gas_tag.py
@@ -1,9 +1,9 @@
 from datetime import datetime, timedelta, timezone
 
-import pytest  # type:ignore[import]
+import pytest
 
+from bdew_datetimes import GERMAN_TIME_ZONE
 from bdew_datetimes.german_strom_and_gas_tag import (
-    GERMAN_TIME_ZONE,
     Division,
     has_no_utc_offset,
     is_gastag_limit,
@@ -87,7 +87,7 @@
 )
 def test_stromtag(
     dt: datetime, expected_is_start_or_end_of_german_stromtag: bool
-):
+) -> None:
     actual = is_stromtag_limit(dt)
     assert actual == expected_is_start_or_end_of_german_stromtag
 
@@ -153,7 +153,9 @@ def test_stromtag(
         ),
     ],
 )
-def test_gastag(dt: datetime, expected_is_start_or_end_of_german_gastag: bool):
+def test_gastag(
+    dt: datetime, expected_is_start_or_end_of_german_gastag: bool
+) -> None:
     actual = is_gastag_limit(dt)
     assert actual == expected_is_start_or_end_of_german_gastag
 
@@ -170,7 +172,9 @@ def test_gastag(dt: datetime, expected_is_start_or_end_of_german_gastag: bool):
         ),
     ],
 )
-def test_has_no_utc_offset(dt: datetime, expected_has_not_utc_offset: bool):
+def test_has_no_utc_offset(
+    dt: datetime, expected_has_not_utc_offset: bool
+) -> None:
     actual = has_no_utc_offset(dt)
     assert actual == expected_has_not_utc_offset
 
@@ -181,7 +185,7 @@ def test_has_no_utc_offset(dt: datetime, expected_has_not_utc_offset: bool):
         pytest.param(datetime(2020, 1, 1, 0, 0, 0, tzinfo=timezone.utc), 0),
     ],
 )
-def test_is_xtag_limit_raises(dt: datetime, division: Division):
+def test_is_xtag_limit_raises(dt: datetime, division: Division) -> None:
     with pytest.raises(NotImplementedError):
         is_xtag_limit(date_time=dt, division=division)
 
@@ -231,6 +235,8 @@ def test_is_xtag_limit_raises(dt: datetime, division: Division):
         ),
     ],
 )
-def test_is_xtag_limit(dt: datetime, division: Division, expected):
+def test_is_xtag_limit(
+    dt: datetime, division: Division, expected: bool
+) -> None:
     actual = is_xtag_limit(dt, division)
     assert actual == expected
diff --git a/tests/test_period.py b/tests/test_period.py
index c8ce385..bae512a 100644
--- a/tests/test_period.py
+++ b/tests/test_period.py
@@ -1,6 +1,6 @@
 from datetime import date
 
-import pytest  # type:ignore[import]
+import pytest
 
 from bdew_datetimes.periods import (
     DayType,
@@ -15,13 +15,13 @@
 )
 
 
-def test_period_instantiation_with_enum():
+def test_period_instantiation_with_enum() -> None:
     period = Period(42, DayType.WORKING_DAY)
     assert period.number_of_days == 42
     assert period.day_type == DayType.WORKING_DAY
 
 
-def test_period_instantiation_with_enum_and_inclusive_end():
+def test_period_instantiation_with_enum_and_inclusive_end() -> None:
     period = Period(
         42, DayType.WORKING_DAY, end_date_type=EndDateType.INCLUSIVE
     )
@@ -38,15 +38,15 @@ def test_period_instantiation_with_enum_and_inclusive_end():
 )
 def test_period_instantiation_with_str(
     number_of_days: int, str_daytype: _DayTyp, expected: Period
-):
+) -> None:
     actual = Period(number_of_days, str_daytype)
     assert actual == expected
 
 
-def test_instantiation_with_invalid_str():
+def test_instantiation_with_invalid_str() -> None:
     with pytest.raises(ValueError):
         _ = Period(
-            42, "Foo"  # type:ignore[argument] # literal is intentionally wrong
+            42, "Foo"  # type:ignore[arg-type] # literal is intentionally wrong
         )
 
 
@@ -72,7 +72,7 @@ def test_instantiation_with_invalid_str():
         ),
     ],
 )
-def test_get_next_working_day(start: date, expected: date):
+def test_get_next_working_day(start: date, expected: date) -> None:
     actual = get_next_working_day(start)
     assert actual == expected
 
@@ -96,7 +96,7 @@ def test_get_next_working_day(start: date, expected: date):
         ),
     ],
 )
-def test_get_previous_working_day(start: date, expected: date):
+def test_get_previous_working_day(start: date, expected: date) -> None:
     actual = get_previous_working_day(start)
     assert actual == expected
 
@@ -226,7 +226,7 @@ def test_get_previous_working_day(start: date, expected: date):
         ),
     ],
 )
-def test_add_frist(start: date, frist: Period, expected: date):
+def test_add_frist(start: date, frist: Period, expected: date) -> None:
     actual = add_frist(start, frist)
     assert actual == expected
 
@@ -350,6 +350,6 @@ def test_add_frist(start: date, frist: Period, expected: date):
 )
 def test_get_nth_working_day_of_month(
     number: int, start: date, month_type: MonthType, expected: date
-):
+) -> None:
     actual = get_nth_working_day_of_month(number, month_type, start)
     assert actual == expected
diff --git a/tox.ini b/tox.ini
index 1402759..08c6e35 100644
--- a/tox.ini
+++ b/tox.ini
@@ -34,11 +34,12 @@ commands =
 # the type_check environment checks the type hints using mypy
 deps =
     -r requirements.txt
+    .[tests]
     .[type_check]
 setenv = PYTHONPATH = {toxinidir}/src
 commands =
-    mypy --show-error-codes src/bdew_datetimes
-    mypy --show-error-codes tests
+    mypy --show-error-codes --strict src/bdew_datetimes
+    mypy --show-error-codes --strict tests
 
 [testenv:format]
 # install isort and black and invoke them on the current folder