diff --git a/main.go b/main.go
index 8ec34bc..e4760c3 100644
--- a/main.go
+++ b/main.go
@@ -388,8 +388,13 @@ func prepareDays(t time.Time) []Day {
 
 	var days []Day
 
-	curDate := t.AddDate(0, 0, -t.Day()+1)
-	preDays := int(curDate.Weekday()) - 1
+	curDate := t.AddDate(0, 0, -(t.Day()-1))
+	preDays := int(curDate.Weekday())
+	if preDays == 0 {
+		preDays = 7
+	}
+	preDays -= 1
+
 	maxDays := daysInMonth(curDate.Year(), curDate.Month())
 
 	endOfMonth := false