diff --git a/tests/mmm/test_fourier.py b/tests/mmm/test_fourier.py
index 64120fc8..d10c18b6 100644
--- a/tests/mmm/test_fourier.py
+++ b/tests/mmm/test_fourier.py
@@ -20,6 +20,7 @@
 import xarray as xr
 
 from pymc_marketing.mmm.fourier import (
+    FourierBase,
     MonthlyFourier,
     YearlyFourier,
     generate_fourier_modes,
@@ -282,71 +283,79 @@ def test_serialization_to_json() -> None:
 
 @pytest.fixture
 def yearly_fourier() -> YearlyFourier:
-    prior = Prior("Normal", dims=("fourier",))
-    return YearlyFourier(n_order=2, prior=prior)
+    prior = Prior("Laplace", mu=0, b=1, dims="fourier")
+    return YearlyFourier(n_order=2, days_in_period=365, prior=prior)
 
 
 @pytest.fixture
 def monthly_fourier() -> MonthlyFourier:
-    prior = Prior("Normal", dims=("fourier",))
-    return MonthlyFourier(n_order=2, prior=prior)
+    prior = Prior("Laplace", mu=0, b=1, dims="fourier")
+    return MonthlyFourier(n_order=2, days_in_period=30, prior=prior)
 
 
-def test_get_default_start_date_none(yearly_fourier) -> None:
-    today = datetime.datetime(2023, 5, 15)
-    expected_start_date = datetime.datetime(2023, 1, 1)
-    assert yearly_fourier.get_default_start_date(today, None) == expected_start_date
+def test_get_default_start_date_none_yearly(yearly_fourier: YearlyFourier):
+    current_year = datetime.datetime.now().year
+    expected_start_date = datetime.datetime(year=current_year, month=1, day=1)
+    actual_start_date = yearly_fourier.get_default_start_date()
+    assert actual_start_date == expected_start_date
 
 
-def test_get_default_start_date_str(yearly_fourier) -> None:
-    today = datetime.datetime.now()
+def test_get_default_start_date_none_monthly(monthly_fourier: MonthlyFourier):
+    now = datetime.datetime.now()
+    expected_start_date = datetime.datetime(year=now.year, month=now.month, day=1)
+    actual_start_date = monthly_fourier.get_default_start_date()
+    assert actual_start_date == expected_start_date
+
+
+def test_get_default_start_date_str_yearly(yearly_fourier: YearlyFourier):
     start_date_str = "2023-02-01"
-    assert (
-        yearly_fourier.get_default_start_date(today, start_date_str) == start_date_str
-    )
+    actual_start_date = yearly_fourier.get_default_start_date(start_date=start_date_str)
+    assert actual_start_date == start_date_str
 
 
-def test_get_default_start_date_datetime(yearly_fourier) -> None:
-    today = datetime.datetime.now()
+def test_get_default_start_date_datetime_yearly(yearly_fourier: YearlyFourier):
     start_date_dt = datetime.datetime(2023, 3, 1)
-    assert yearly_fourier.get_default_start_date(today, start_date_dt) == start_date_dt
+    actual_start_date = yearly_fourier.get_default_start_date(start_date=start_date_dt)
+    assert actual_start_date == start_date_dt
 
 
-def test_get_default_start_date_invalid_type(yearly_fourier) -> None:
-    today = datetime.datetime.now()
-    invalid_start_date = 12345  # Invalid type
+def test_get_default_start_date_invalid_type_yearly(yearly_fourier: YearlyFourier):
+    invalid_start_date = 12345  # Invalid type again
     with pytest.raises(TypeError) as exc_info:
-        yearly_fourier.get_default_start_date(today, invalid_start_date)
+        yearly_fourier.get_default_start_date(start_date=invalid_start_date)
     assert "start_date must be a datetime.datetime object, a string, or None" in str(
         exc_info.value
     )
 
 
-def test_get_default_start_date_none_monthly(monthly_fourier) -> None:
-    today = datetime.datetime(2023, 5, 15)
-    expected_start_date = datetime.datetime(2023, 5, 1)
-    assert monthly_fourier.get_default_start_date(today, None) == expected_start_date
-
-
-def test_get_default_start_date_str_monthly(monthly_fourier):
-    today = datetime.datetime.now()
+def test_get_default_start_date_str_monthly(monthly_fourier: MonthlyFourier):
     start_date_str = "2023-06-15"
-    assert (
-        monthly_fourier.get_default_start_date(today, start_date_str) == start_date_str
+    actual_start_date = monthly_fourier.get_default_start_date(
+        start_date=start_date_str
     )
+    assert actual_start_date == start_date_str
 
 
-def test_get_default_start_date_datetime_monthly(monthly_fourier) -> None:
-    today = datetime.datetime.now()
+def test_get_default_start_date_datetime_monthly(monthly_fourier: MonthlyFourier):
     start_date_dt = datetime.datetime(2023, 7, 1)
-    assert monthly_fourier.get_default_start_date(today, start_date_dt) == start_date_dt
+    actual_start_date = monthly_fourier.get_default_start_date(start_date=start_date_dt)
+    assert actual_start_date == start_date_dt
 
 
-def test_get_default_start_date_invalid_type_monthly(monthly_fourier) -> None:
-    today = datetime.datetime.now()
+def test_get_default_start_date_invalid_type_monthly(monthly_fourier: MonthlyFourier):
     invalid_start_date = [2023, 1, 1]
     with pytest.raises(TypeError) as exc_info:
-        monthly_fourier.get_default_start_date(today, invalid_start_date)
+        monthly_fourier.get_default_start_date(start_date=invalid_start_date)
     assert "start_date must be a datetime.datetime object, a string, or None" in str(
         exc_info.value
     )
+
+
+def test_fourier_base_instantiation():
+    with pytest.raises(TypeError) as exc_info:
+        FourierBase(
+            n_order=2,
+            days_in_period=365,
+            prior=Prior("Laplace", mu=0, b=1, dims="fourier"),
+        )
+    assert "Can't instantiate abstract class FourierBase" in str(exc_info.value)