By Rabbi Chaim Chazzan
A woman baked challah and realized on Shabbos that she had forgotten to separate challah. May challah be taken on Shabbos?
Lechatchilah, challah should be taken before baking, as the possuk says to separate it from dough. If one forgot to be mafrish challah before baking, one should separate challah afterwards. A bracha is made then as well, provided that the dough had the required amount.
It is forbidden to eat from bread from which challah had not been taken (tevel). The chachomim prohibited separating challah on Shabbos or Yom Tov, since one is making something previously unfit (for consumption) become suitable for use, similar to fixing a utensil. One may, however, take challah from dough which was kneaded on Yom Tov itself. The chachomim rendered this permissible because on Yom Tov one is allowed to knead and bake, and only once challah has been taken can it be eaten.
Min hatorah the obligation to separate challah only applies in Eretz Yisroel. The chachomim instituted for challah to be taken in chutz la’aretz as well, in order for the concept of challah not to be forgotten. [This concern is more relevant by challah than by trumah and maiser, because the latter only apply to those engaged in agriculture, whereas challah pertains to any person baking bread, hence the concern that people might get used to baking bread without taking challah is more pertinent.]
Regarding chutz la’aretz, the chachomim were lenient, that one may eat bread before challah is taken, leave over a piece and separate challah from that which is left over. However, one should not make a practice of doing this, lest one forgets to leave some over and then retroactively one would have eaten tevel.
In light of the above, the din will depend on the location of this shailah: If it took place in Eretz Yisroel, then the challah loaves would be forbidden until Motzo’ei Shabbos when challah may be taken. However, in chutz la’aretz one should set aside a slice from which to separate challah after Shabbos, and the rest of the challah may then be eaten.
 שו”ע יו”ד סי’ שכז סעי’ ה.
 שוע”ר סי’ רס ס”ה, וראה שם סי’ שלט ס”ז
שוע”ר סי’ תקו ס”ה
 מסכת חלה פ”ד מ”ח, רמב”ם בכורים פ”ה ה”ז טושו”ע יו”ד שכב, שכג
 תוס’ בקדושין לו ע”ב ד”ה כל, וראה תוס’ יו”ט חלה שם ד”ה שתי חלות, וע”ע אנצקלופדיה תלמודית ערך חלה או”ק טז
 שו”ע יו”ד סי’ שכג ס”א
 שוע”ר סי’ רס ס”ה. וראה ג”כ שם סי תנז סי”ב
 שו”ע שם