בחדש הראשון [AND THE LORD SPAKE UNTO MOSES] IN THE FIRST MONTH — The section which appears at the commencement of this Book was not spoken before Eyar (the second month; cf. Numbers 1:1): you learn, therefore, that there is no “earlier” or “later” (no chronological order) in the Torah. But why, indeed, did not Scripture open the Book with this section? Because it implies something disparaging to Israel (Sifrei Bamidbar 64:1) — that during all the forty years they were in the wilderness they offered only this single Passover sacrifice (Tosafot on Kiddushin 38b s. v. הואיל)
It is explained that they remained there for 11 months and this is a degrading fact since had they not sinned they would have quickly entered The Land. And the Torah already mentioned events that happened in the second month and this is a source that "there is no chronological order" in the Torah. And Ramban writes that the reason for the delay is because the Fourth Book of the Torah is to recall the specific mitzvot that were commanded in the Wilderness of Sinai for that time only and the Torah wanted to complete all of the issues related to the mishkan and how it operated for the entire duration of the time in the Desert. So first the Torah mentions the banners and the arrangement of the tents and then it mentions those who served in the mishkan and the establishment of shifts to carry the mishkan and it mentions the offerings brought by the princes of the tribes and the dedication ceremony. And only after does it mention the warning not to forget the mitzvah of Pesach...