וַיְהִ֣י הַיּ֔וֹם וַיִּזְבַּ֖ח אֶלְקָנָ֑ה וְנָתַ֞ן לִפְנִנָּ֣ה אִשְׁתּ֗וֹ וּֽלְכָל־בָּנֶ֛יהָ וּבְנוֹתֶ֖יהָ מָנֽוֹת׃
Und es geschah an einem Tag, an dem Elkanah opferte, dass er seiner Frau Peninnah und all ihren Söhnen und Töchtern Portionen gab;
Rashi on I Samuel
It happened on the day. It happened on the festive day.
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."