Q. I just read in an authoritative Yom Tov sefer and luach that Parshat Bamidvar is always before Shavuoth, wasn’t last year Nasso before?

A. This sefer was probably quoting Shulchan Oruch (O.H. 428, 4) that very emphatically rules “leolam korin” we always read (the chapter) of Bamidbar before Shavuoth. However, as Biur Halocho (ibid.) points out, occasionally it is Nasso not Bamidbar (12 times in the last one hundred years outside of Israel, 19 times there, see question 86 in this forum). Poiskim indeed wonder why use the expression “leolam” when it is not really always so (Bein Pesach L’shavuos 9). Biur Halocho (ibid.) quotes the Levush, that places the emphasis on separating the curses of the tochacha of parshas Bechukosai from the judgment of Shavuoth (on the fruits of trees), so as not to give an arraigning opportunity to the accuser in a day of judgment. Parshas Bamidbar indeed always serves as a separating instrument between the parshios before Shavuos.

Some authorities explain that parshas Bamidbar is exceptional not only as a disconnecting factor but also because of the different messages an teachings necessary for the receiving of the Torah.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a