diff --git a/freedays/README.md b/freedays/README.md
index dc0e794..4e38345 100644
--- a/freedays/README.md
+++ b/freedays/README.md
@@ -52,6 +52,15 @@ app launch.
 | **Releasing refunds only the future** | Give back a day that has not arrived and it returns to the pool. Give back today, and it does not: those hours were already unguarded. |
 | **Silent** | No notification, no low-budget warning, no prompt when a gate fires. You find out by asking. |
 
+## Weekly workdays
+
+`freedays.WORKDAYS` (`{1, 2, 3}`, Tue/Wed/Thu in `date.weekday()` numbering),
+`NON_WORKDAYS` and `is_workday(day=None)` are the one definition of the weekly
+workdays every gate app reads — screen-locker's relaxed days, leetcode-guard's
+cheap days, wake-alarm's ramp hold and workday stick. Import it; never write
+the set out again. Dart/Kotlin code that cannot import it keeps a literal copy
+guarded by a drift test against this package.
+
 ## Free days are not sick days
 
 screen-locker's sick days stay exactly where they are, with their rolling
diff --git a/freedays/freedays/__init__.py b/freedays/freedays/__init__.py
index 220f764..8a94798 100644
--- a/freedays/freedays/__init__.py
+++ b/freedays/freedays/__init__.py
@@ -52,9 +52,12 @@ from freedays._errors import (
 from freedays._model import FreeDay
 from freedays._paths import Paths
 from freedays._sync import SyncUnavailableError, sync, sync_quietly
+from freedays._workdays import NON_WORKDAYS, WORKDAYS, is_workday
 
 __all__ = [
     "DEFAULT_ANNUAL_BUDGET",
+    "NON_WORKDAYS",
+    "WORKDAYS",
     "AlreadyFreeError",
     "BudgetExhaustedError",
     "FreeDay",
@@ -68,6 +71,7 @@ __all__ = [
     "free_days_in_year",
     "is_exhausted",
     "is_free_day",
+    "is_workday",
     "load",
     "lookup",
     "mark",
diff --git a/freedays/freedays/_workdays.py b/freedays/freedays/_workdays.py
new file mode 100644
index 0000000..6aa0717
--- /dev/null
+++ b/freedays/freedays/_workdays.py
@@ -0,0 +1,38 @@
+# Copyright (c) 2026 Krzysztof Rudnicki
+"""The weekly workdays: Tuesday, Wednesday and Thursday.
+
+The one definition every gate app reads, so "which days are workdays" cannot
+drift between them. Consumers each give it their own meaning: screen-locker
+relaxes the lock, leetcode-guard charges the cheap price, wake-alarm's ramp
+holds the wake time instead of stepping it earlier, and a missed workday
+morning costs tomorrow its leniency.
+
+Languages that cannot import this package (the Dart and Kotlin sides of the
+phone apps) keep a literal copy and a test that fails when it drifts from
+this one.
+"""
+
+from __future__ import annotations
+
+from datetime import date
+from typing import Final
+
+from freedays._day import today
+
+#: Python ``date.weekday()`` numbering: Mon=0 ... Sun=6.
+WORKDAYS: Final[frozenset[int]] = frozenset({1, 2, 3})
+
+#: The complement: Monday, Friday, Saturday and Sunday.
+NON_WORKDAYS: Final[frozenset[int]] = frozenset(range(7)) - WORKDAYS
+
+
+def is_workday(day: date | None = None) -> bool:
+    """Return whether *day* (default: today, local) is a weekly workday.
+
+    Args:
+        day: The date to classify; ``None`` means today.
+
+    Returns:
+        True on Tuesday, Wednesday and Thursday.
+    """
+    return today(day).weekday() in WORKDAYS
diff --git a/freedays/freedays/tests/test_workdays.py b/freedays/freedays/tests/test_workdays.py
new file mode 100644
index 0000000..5346335
--- /dev/null
+++ b/freedays/freedays/tests/test_workdays.py
@@ -0,0 +1,39 @@
+# Copyright (c) 2026 Krzysztof Rudnicki
+"""The weekly workdays: the one Tue/Wed/Thu definition every app reads."""
+
+from __future__ import annotations
+
+from datetime import date
+
+import pytest
+
+import freedays
+
+
+def test_workdays_are_tuesday_to_thursday() -> None:
+    assert freedays.WORKDAYS == frozenset({1, 2, 3})
+
+
+def test_non_workdays_are_the_complement() -> None:
+    assert freedays.NON_WORKDAYS == frozenset({0, 4, 5, 6})
+    assert not freedays.WORKDAYS & freedays.NON_WORKDAYS
+
+
+@pytest.mark.parametrize(
+    ("day", "expected"),
+    [
+        (date(2026, 9, 28), False),  # Monday
+        (date(2026, 9, 29), True),  # Tuesday
+        (date(2026, 9, 30), True),  # Wednesday
+        (date(2026, 10, 1), True),  # Thursday
+        (date(2026, 10, 2), False),  # Friday
+        (date(2026, 10, 3), False),  # Saturday
+        (date(2026, 10, 4), False),  # Sunday
+    ],
+)
+def test_is_workday(day: date, expected: bool) -> None:
+    assert freedays.is_workday(day) is expected
+
+
+def test_is_workday_defaults_to_today() -> None:
+    assert freedays.is_workday() is (date.today().weekday() in {1, 2, 3})
diff --git a/freedays/pyproject.toml b/freedays/pyproject.toml
index 2b720b3..e9707e8 100644
--- a/freedays/pyproject.toml
+++ b/freedays/pyproject.toml
@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "freedays"
-version = "0.1.0"
+version = "0.3.0"
 description = "Shared pre-declared free-day pool: one global list of days on which every gate app stands down"
 requires-python = ">=3.14"
 # Deliberately NOT gatelock. Its __init__ imports tkinter for the lock-window