"And it came to pass the day" (1 Samuel 1:4). Rabbi Yehoshua ben Levi says: This refers to the day of Shavuot [the Feast of Weeks, when Elkanah went up to offer his sacrifice].
And It Came to Pass the Day of Shavuot
Yalkut Shimoni on Nach 77:7
ויהי היום. ר' יהושע בן לוי אומר (ג) זה יומו של עצרת: