diff --git a/tests/test_transforms.py b/tests/test_transforms.py
index 2f1b2d0d..f3b2ffe2 100644
--- a/tests/test_transforms.py
+++ b/tests/test_transforms.py
@@ -123,7 +123,11 @@ class TestEventWindow(TestCase):
             # Record the length of the window after each event.
             lengths.append(len(window.ticks))
 
-        assert lengths == [1, 2, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3]
+        # The window stretches out during the weekend because we wait
+        # to drop events until the weekend ends. The last window is
+        # briefly longer because it doesn't complete a full day.  The
+        # window then shrinks once the day completes
+        assert lengths == [1, 2, 3, 3, 3, 4, 5, 5, 5, 3, 4, 3]
         assert window.added == events
         assert window.removed == events[:-3]
 
@@ -142,7 +146,7 @@ class TestEventWindow(TestCase):
             # Record the length of the window after each event.
             lengths.append(len(window.ticks))
 
-        assert lengths == [1, 2, 2, 2, 2]
+        assert lengths == [1, 2, 3, 3, 2]
         assert window.added == events
         assert window.removed == events[:-2]
 
diff --git a/zipline/transforms/utils.py b/zipline/transforms/utils.py
index ea8b7612..4ee0a999 100644
--- a/zipline/transforms/utils.py
+++ b/zipline/transforms/utils.py
@@ -239,6 +239,9 @@ class EventWindow(object):
         # adding new ticks.
         self.handle_add(event)
 
+        if self.market_aware:
+            self.add_new_holidays(event.dt)
+
         # Clear out any expired events. drop_condition changes depending
         # on whether or not we are running in market_aware mode.
         #
@@ -254,13 +257,33 @@ class EventWindow(object):
             # behavior for removing ticks.
             self.handle_remove(popped)
 
-    def out_of_market_window(self, oldest, newest):
-        # Find number of unique days in window
-        # Note that this assumes that each day we received an
-        # event is a trading day.
-        unique_dts = set([event.dt.date() for event in self.ticks])
+    def add_new_holidays(self, newest):
+        # Add to our tracked window any untracked holidays that are
+        # older than our newest event. (newest should always be
+        # self.ticks[-1])
+        while len(self.all_holidays) > 0 and self.all_holidays[0] <= newest:
+            self.cur_holidays.append(self.all_holidays.popleft())
 
-        return len(unique_dts) > self.window_length
+    def drop_old_holidays(self, oldest):
+        # Drop from our tracked window any holidays that are older
+        # than our oldest tracked event. (oldest should always
+        # be self.ticks[0])
+        while len(self.cur_holidays) > 0 and self.cur_holidays[0] < oldest:
+            self.cur_holidays.popleft()
+
+    def out_of_market_window(self, oldest, newest):
+        self.drop_old_holidays(oldest)
+        calendar_dates_between = (newest.date() - oldest.date()).days
+        holidays_between = len(self.cur_holidays)
+        trading_days_between = calendar_dates_between - holidays_between
+
+        # "Put back" a day if oldest is earlier in its day than newest,
+        # reflecting the fact that we haven't yet completed the last
+        # day in the window.
+        if oldest.time() > newest.time():
+            trading_days_between -= 1
+
+        return trading_days_between >= self.window_length
 
     def out_of_delta(self, oldest, newest):
         return (newest - oldest) >= self.delta