Likutei Sichos – Vol 23 p 62 ff 1) The idea of אין וקדם ומאוחר בסדר התורה is derived form here. The Rebbe explains this nuance and the very beginning of the Sicha. The Korban Pesach brought in 5749 that is placed in Parshas Biha’alosecha rather than in the beginning of the Chumash Bamidbar. It is not put in the beginning because (though a wonderful event, particularly the Pesach Sheini aspect) but it exposes the weakness of the Jewish people for not demanding that it be bright in the remaining years in the Midbar.
