On Shabbat, the sages forbade exfoliate challah, and a pastry from which you forgot to take challah is forbidden even to move it because it is muktzeh.
Outside of Israel, the sages permitted eating bread and leaving a piece for the end and then taking challah. Therefore, if you forgot to set aside challah abroad, you can eat the bread and leave a little for after Shabbat.
On Yom Tov, it is allowed to separate a challah from a pastry that was made on Yom Tov itself.
If you remembered before the night that you forgot to take challah and you need the challahs to eat on Shabbat, you are allowed to set aside even though it was already sunset and lit Shabbat candles, as long as the night has not yet arrived.
Everything that Avraham Avinu himself did for the guests, God did for Abraham’s sons himself, and everything that Abraham did by a messenger, God sent through a messenger. Abraham was standing next to the guests under the tree – God, too, stood on with a pillar of cloud and fire near the Israelites in the desert. Abraham gave them bread, God gave them break – Man. Abraham brought them meat, and Ashem gave Coturnix to Israel, but Abraham asked his son to give them water, so the Ashem asked Moshe to give them water.
It follows that if our Abraham gave the guests water himself, God would also give water himself, and Moshe Rabbeinu would not be punished because of the rock and would enter the land of Israel and build the Temple, and it was never destroyed, and Israel was not in exile for two thousand years!
If a guest comes to you, and you ask your daughter to bring her a glass of juice or a piece of cake, does it matter that you asked the girl and did not get up by yourself? Certainly not, what difference does it make if I did or did not do a small act… but see that it does matter! True, here in our world we do not see differences, but in the sky, they see things differently, in the sky, there isn’t such a thing as a “little act” – even one act can change the history of mankind! Did Abraham know that because of a glass of water the whole world would change in 400 years?
So I do not say that from now on you have to get up and do everything yourself, but at least to know, that certainly the mitzvot and chesed that you do by yourself are worth much more, and you also sanctify yourself, because with your own hands you do mitzvot, and also show your friend how much you care about her, and that it is pleasant for you to bother for her, and when you make her happy, is it possible that God will not please you back?