Metzudat David on I Samuel
And it was on that day: Meaning to say, when the day came that he would bring his sacrifices, Elkanah would sacrifice peace-offerings and he would give portions to Peninah, etc. from the sacrifice of the peace-offerings.
Radak on I Samuel
And it was on that day: The day of the set time; and likewise did Yonatan translate it [in the Targum] as, "the day of the set time." And in the homiletical teachings: Rabbi Yehoshua be Levi said, "This was the day of Shavuot."