Are you allowed to daven maariv on 1st night Shavuoth before Tzais Hakachovim?

Shalom, Thank you for your question. The Shlah wrote that one should not bring in the festival of Shavuot early, as one would miss out on having 49 complete weeks of counting the Omer. The Torah writes that we should count "Seven complete weeks" – and he understood "complete" to mean that we should not start the festival until the last day of the counting (day 49, the day before Shavuot) is completed by the end of the day, and three stars coming out at night. This is quoted by many major later day codifiers (see Mishna Brurah 504, 1). This is certainly the correct thing to do. However, as there are lenient opinions, in a time of need, especially in places where the festival comes in very late at night, those who bring in the festival early (after "plag hamincha") should not be rebuked, as they have opinions upon who to rely. (See B'nie Banim of Rav Y.H. Henkin shlitah, volume 3, 2). Blessings.