This year, Shavuot (the Jewish festival behind the Christian Pentecost Sunday) is May 14-15 (sundown to sundown). You can read more Jews for Jesus articles on this holiday here. Beyond that, David Brickner and Rich Robinson have written a very thorough guide to understanding this holiday and how it relates to believers in Jesus. Christ in the Feast of Pentecost is written not just to inform, but to inspire you and even provide practical help for believers who want to celebrate this holiday today. You can order the book here.