diff --git a/goocalendar/_calendar.py b/goocalendar/_calendar.py
index 9114d16b18d666ac65e9b39b93339919b126c4cc_Z29vY2FsZW5kYXIvX2NhbGVuZGFyLnB5..13416f326c1f4a14b5f6390e68fd077e4da4142e_Z29vY2FsZW5kYXIvX2NhbGVuZGFyLnB5 100644
--- a/goocalendar/_calendar.py
+++ b/goocalendar/_calendar.py
@@ -282,7 +282,7 @@
                 the_body_color = self.props.body_color
 
             # Draw.
-            box = self.days[weekno * 7 + dayno + 1]
+            box = self.days[weekno * 7 + dayno]
             box.x = timeline_w
             box.y = 0
             box.width = self._day_width - 2