diff --git a/calendar/osism.html b/calendar/osism.html
index 288690b..8498253 100644
--- a/calendar/osism.html
+++ b/calendar/osism.html
@@ -109,6 +109,7 @@ <h3 id="popupTitle">Event Title</h3>
 
       const calendar = new FullCalendar.Calendar(calendarEl, {
         initialView: 'dayGridMonth',
+	firstDay: 1,
         headerToolbar: {
           left: 'prev,next today',
           center: 'title',