See Moishe Dovid Lebovits, "Lag B'Omer," 5 Towns Jewish Times:
Night or Day. Many have the custom to
make the bonfires (and dancing) on the
night of Lag B’omer. Others say that
the simcha should start at day.85
However, it seems that the minhag is
to conduct the bonfire and dancing at
night all over the world, not only in
Meron.86 However, music in the
absence of a bonfire is prohibited
until the next day after neitz
85: Harav Yisrael Belsky Shlita, opinion of Harav Korelitz Shlita quoted in Doleh U’mashka page 194:footnote 520.
86: Harav Yisrael Belsky Shlita, see Mekadesh Yisrael 103, Chut Shani Shabbos 4:page 380, Doleh U’mashka pages 194-195 footnote 521. The Halichos Shlomo Moadim 2:page 362:footnote 23 says dancing is permitted on Lag B’omer even if it is not for a devar mitzvah (refer to Ben Pesach L’Shavuos 15:4:footnote 4). The Avnei Yushpei 1:98 is not convinced that dancing is permitted at all after Lag B’omer.
87: Refer to Doleh U’mashka page 194:footnote 520, Mekadesh Yisrael 103.