Parashas Masei · Passuk 33:3 · TT note 1
Chabad.org — English translation + Rashi
Torah Temimah points to Yerushalmi Rosh Hashana 2:8 (TT note on “בַּחֲמִשָּׁה עָשָׂר יוֹם לַחֹדֶשׁ”).
English: “He made the moon for festivals; the sun knows its setting” — only after the sun’s setting do you reckon the moon for the appointed times. Rabbi Berachya: the verse that they journeyed on the fifteenth of the first month shows that when the sun sets on the festival night there are fourteen settings in the month — you count the moon from sunset.
Custom / calendar derivation from the wording of the journey date (how months are counted from sunset).
The departure on the fifteenth of Nisan teaches that lunar months are counted from sunset, not from the midday molad — the moon’s festivals follow the sun’s setting.