diff --git a/jcal/src/Calendar.tsx b/jcal/src/Calendar.tsx
index 9baf68b..96931db 100644
--- a/jcal/src/Calendar.tsx
+++ b/jcal/src/Calendar.tsx
@@ -54,7 +54,7 @@ function Day({year, month, day, todayJDN}: DateProps & { todayJDN: number }): JS
 function Month({year, month, todayJDN}: MonthProps & { todayJDN: number }): JSX.Element {
     const decadeHeads = weekdayNames.map((name, i) => <WeekdayName key={i} name={name}/>);
     const firstJDN = julianJDN(year, month, 1);
-    const firstWeekday = (firstJDN % 8 + 8) % 7;
+    const firstWeekday = (firstJDN % 7 + 8) % 7;
     const daysTotal = julianMonthDays(year, month);
     return <div className="Month">
         <div className="Month-weekdayHead">{decadeHeads}</div>