The Mishna Berura (494:1) writes that Arvit should not be recited before nightfall, so that forty-nine complete days of the Omer pass before we celebrate the fiftieth day, Shavuot. Chacham Ovadia Yosef, however, ruled that one may be lenient and begin the holiday before Tzet Ha'kochavim, if waiting poses difficulty for one's family. Likewise, Chacham Baruch Ben-Chaim would recite Arvit and Kiddush before Tzet Ha'kochavim on the night of Shavuot, explaining that he wanted to ensure that the children can remain awake to participate in the Yom Tov celebration.