Komentarz do Powtórzonego Prawa 16:1

שמור את חדש האביב WATCH THE MONTH OF ABIB — This means: Before it comes watch whether it will be capable of producing ripe ears (אביב), so that one may offer the Omer meal-offering during it, and if not (i.e. if you observe that the ears will not be ripe by the 16th of Nisan), then intercalate the year (i.e. add a month to the winter-period, so that the month Abib falls later than it otherwise would, by which time the ears will be ripened) (cf. Sanhedrin 11b and Note 4 on p. 191 of Leviticus in the Silbermann edition of the Pentateuch).

KEEP THE MONTH OF AVIV. The commandment of the festivals [mentioned here] is explanatory, for He has already mentioned it. Now, in the Book of Leviticus He mentioned with reference to the festivals, And ye shall bring an offering made by fire unto the Eternal,³¹⁴Leviticus 23:8. but here he did not mention them [the offerings] at all. Instead he commanded Israel to ascend on their account [to celebrate the festivals] to the place which He will choose, and rejoice before Him. For just as he commanded that one bring the Second Tithe before G-d,³¹⁵Above, 14:23. and also the firstling,³¹⁶Ibid., 15:19. and eat them there in the place that G-d will choose, he added [here] to make it clear that they will all be further obligated to come before G-d and celebrate before Him on three festivals and rejoice with the peace-offerings that they will offer before Him. Now he did not mention the times of the Festivals of Unleavened Bread and Tabernacles on which dates they occur, but he briefly mentioned their months, for He had already stated everything there.³¹⁷Leviticus 23:4-43.

שמור את חודש האביב, make sure that the month of Nissan always occurs in the spring, by manipulating the calendar, if necessary and inserting an extra month of Adar. By doing this the solar orbit (year) and the lunar orbits, (12 moons equaling a year) can be brought into line with each other.