Forgetting to Separate Challah Before Shabbos

I forgot to separate challah before Shabbos from the bread that I baked. What should I do? Am I permitted to separate challah on Shabbos?

Shulchan Aruch (OC 339:4) writes that one is not permitted to separate Terumos and Ma’aseros (various tithes) on Shabbos. Included in this prohibition is separating challah. Mishnah Berurah (261:4) explains that Chazal forbade separating tithes on Shabbos and Yom Tov since this “fixes” the foods and makes them useable. “Fixing” food is forbidden on Shabbos and Yom Tov, since it resembles the melacha of tikun maneh (the activity of fixing utensils). However, outside of Israel, one may eat bread before challah is separated, provided that challah will be separated later. Therefore, if on Shabbos one realizes that they did not separate challah, they should make sure to leave over a slice of bread, so they can separate challah after Shabbos.

The Gerald & Karin Feldhamer OU Kosher Halacha Yomis is dedicated to the memory of Rav Yisroel Belsky, zt"l, who served as halachic consultant for OU Kosher for more than 28 years; many of the responses in Halacha Yomis are based on the rulings of Rabbi Belsky. Subscribe to the Halacha Yomis daily email here.