I've found different sources like the Mishnah, Babylonian Talmud (tractate Rosh HaShanah), Jerusalem Talmud, Tosefta etc., talking about the Minim and the Boethusians who wanted to fix the festival of Shavuot the day after the Shabbat, i.e. on Sunday.
I would really like to know what are the earliest sources known about this topic, and what's the whole story behind this.
Please, feel free to edit.