Parashas Bo · Shemos 12:18 · TT note 3
Torah Temimah on Shemos 12:18, note 3 (on הָאֶחָד וְעֶשְׂרִים לַחֹדֶשׁ בָּעָרֶב) points to Rosh Hashanah 20b.
Sefaria — Exodus 12:18
Sefaria — Torah Temimah on Exodus 12:18
Sefaria — Rosh Hashanah 20b
E-Daf — Rosh Hashanah 20b
English: When R’ Zeira went up [to Eretz Yisrael], he sent to them: there must be night and day of the [new] month… Reish Lakish said [the source is]: “until the twenty-first day of the month in the evening.”
Kiddush ha-chodesh requires a full night-and-day unit of the month.
This verse’s “evening” endpoint is used to show that a calendar day of the month includes night and day together.