On Shavuos night, the minhag is to wait until it is night – Tzeis Hakochavim – to daven Maariv in order to have 49 complete days. (Shulchan Aruch w/Mishnah Brurah 494:1).
Likewise, it is preferable for women to wait until it is night to light candles and say Shehechiyanu on Shavuos night. (Luach R’ Yechiel Michel Tukatchinsky, Piskei Tshuvos 494:2)
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We are not “waiting to count.” We don’t count on the night of Shavuos. The Torah says “tispiru chamishim yom”, but we do NOT count the 50th day.
See Michtav M’Eliyahu and other Seforim for why.
The photo is of Breuer’s, where they daven Maariv before nacht.