diff --git a/README.md b/README.md
index 4cc7357..5fe56ec 100644
--- a/README.md
+++ b/README.md
@@ -78,7 +78,8 @@ assert MonthEvents('active', now.year, now.month).has_events_marked() == True
 How many users have been active this week?:
 
 ```python
-print(len(WeekEvents('active', now.year, now.isocalendar()[1])))
+iso_year, iso_week, _ = now.isocalendar()
+print(len(WeekEvents('active', iso_year, iso_week)))
 ```
 
 Iterate over all users active this week:
@@ -111,7 +112,8 @@ MonthEvents('active').from_date(now) == MonthEvents('active', now.year, now.mont
 Get the list of these users (user ids):
 
 ```python
-print(list(WeekEvents('active', now.year, now.isocalendar()[1])))
+iso_year, iso_week, _ = now.isocalendar()
+print(list(WeekEvents('active', iso_year, iso_week)))
 ```
 
 There are special methods `prev` and `next` returning "sibling" events and
diff --git a/bitmapist/__init__.py b/bitmapist/__init__.py
index 37757f3..6bebafc 100644
--- a/bitmapist/__init__.py
+++ b/bitmapist/__init__.py
@@ -46,7 +46,8 @@
 
 How many users have been active this week?::
 
-    print len(WeekEvents('active', now.year, now.isocalendar()[1]))
+    iso_year, iso_week, _ = now.isocalendar()
+    print len(WeekEvents('active', iso_year, iso_week))
 
 Perform bit operations. Which users that have been active last month are still active this month?::
 
diff --git a/bitmapist/cohort/__init__.py b/bitmapist/cohort/__init__.py
index 91d4e29..73453d1 100644
--- a/bitmapist/cohort/__init__.py
+++ b/bitmapist/cohort/__init__.py
@@ -384,7 +384,8 @@ def _day_events_fn(key: str, date: date, system: str):
 
 def _weeks_events_fn(key: str, date: date, system: str):
     cls = WeekEvents
-    cls_args = (date.year, date.isocalendar()[1], system)
+    iso_year, iso_week, _ = date.isocalendar()
+    cls_args = (iso_year, iso_week, system)
     return _dispatch(key, cls, cls_args)
 
 
diff --git a/test/test_bitmapist.py b/test/test_bitmapist.py
index f255087..7d264cb 100644
--- a/test/test_bitmapist.py
+++ b/test/test_bitmapist.py
@@ -26,8 +26,9 @@ def test_mark_with_diff_days():
     assert 124 not in MonthEvents("active", now.year, now.month)
 
     # Week
-    assert 123 in WeekEvents("active", now.year, now.isocalendar()[1])
-    assert 124 not in WeekEvents("active", now.year, now.isocalendar()[1])
+    iso_year, iso_week, _ = now.isocalendar()
+    assert 123 in WeekEvents("active", iso_year, iso_week)
+    assert 124 not in WeekEvents("active", iso_year, iso_week)
 
     # Day
     assert 123 in DayEvents("active", now.year, now.month, now.day)
diff --git a/test/test_cohort.py b/test/test_cohort.py
index 918ee23..9a034e5 100644
--- a/test/test_cohort.py
+++ b/test/test_cohort.py
@@ -1,9 +1,9 @@
-from datetime import datetime, timedelta, timezone
+from datetime import date, datetime, timedelta, timezone
 
 import pytest
 
 from bitmapist import mark_event
-from bitmapist.cohort import get_dates_data
+from bitmapist.cohort import _weeks_events_fn, get_dates_data
 
 
 @pytest.fixture
@@ -35,22 +35,16 @@ def events():
         ("active", None, "active", None, [2, 100, 50]),
         ("active", None, "unknown", None, [2, "", ""]),
         ("unknown", None, "active", None, [0, "", ""]),
-
         # Tests with select1b (AND conditions for select1)
         ("signup", "active", "active", None, [2, 100, 50]),
-
         # Tests with both select1b and select2b
         ("task1", "task2", "task2", "task1", [2, 100, 100]),
-
         # When select1 has no events but select1b has events, result should be 0
         ("unknown", "active", "active", None, [0, "", ""]),
-
         # When select1 has events but select2 AND select2b results in 0
         ("active", None, "unknown", "active", [2, "", ""]),
-
         # When select1 has events but select1b has no events (no overlap)
         ("active", "unknown", "active", None, [0, "", ""]),
-
         # When select2 has events but select2b has no events (no overlap)
         ("active", None, "active", "unknown", [2, "", ""]),
     ],
@@ -67,3 +61,30 @@ def test_cohort(select1, select1b, select2, select2b, expected, events):
         num_of_rows=1,
     )
     assert r[0][1:] == expected
+
+
+def test_weeks_events_fn_iso_year_boundary():
+    """Test that _weeks_events_fn uses ISO year, not calendar year.
+
+    At year boundaries, a date's calendar year can differ from its ISO year.
+    For example, Dec 30, 2024 is in ISO week 1 of 2025.
+    """
+    # Dec 30, 2024 is a Monday in ISO week 1 of 2025
+    dec_30_2024 = date(2024, 12, 30)
+    iso_year, iso_week, _ = dec_30_2024.isocalendar()
+    assert iso_year == 2025  # Verify our test date assumption
+    assert iso_week == 1
+
+    event = _weeks_events_fn("test_event", dec_30_2024, "default")
+    # Should use ISO year (2025), not calendar year (2024)
+    assert event.year == 2025
+    assert event.week == 1
+
+    # Jan 1, 2025 is also in ISO week 1 of 2025
+    jan_1_2025 = date(2025, 1, 1)
+    event2 = _weeks_events_fn("test_event", jan_1_2025, "default")
+    assert event2.year == 2025
+    assert event2.week == 1
+
+    # Both dates should produce the same week event
+    assert event.redis_key == event2.redis_key