Sforno on Exodus 23:15/ספורנו על שמות כ״ג:ט״ו

1את חג המצות תשמור, the word תשמור is meant in the same sense as שמור את חודש האביב ועשית פסח, “observe the month of spring to prepare the Passover.” (Deut. 1,1.) למועד חודש האביב, a subtle hint to the authorities to ensure that the calendar dates given for this festival coincide with the season of spring. This may be accomplished by an extra day in certain months, or in more drastic situations by the insertion of a second month of Adar.