Gen 17:21 "But My covenant I establish with Yitschaq, whom Sarah is to bear to you at this set time ("mo’ad" – word normally used to refer to a feast) next year."

Gen 18:10 And He said, "I shall certainly return to you according to the time of life (Literally: "when this time becomes alive again"), and see, Sarah your wife is to have a son!" And Sarah was listening in the tent door which was behind him.

Gen 18:14 "Is any matter too hard for Yahweh? At the appointed time ("mo’ad") I am going to return to you, according to the time of life (Literally: "when this time becomes alive again"), and Sarah is to have a son."

Gen 19:1 And the two messengers came to Sedom in the evening, and Lot was sitting in the gate of Sedom. And when Lot saw them, he rose up to meet them, and he bowed himself with his face toward the ground,

Gen 19:2 and he said, "Look, please my masters, please turn in to your servant’s house and spend the night, and wash your feet, and rise early and go your way." And they said, "No, but let us spend the night in the open square."

Gen 19:3 But he urged them strongly, and they turned in to him and came into his house. And he made them a feast, and baked unleavened bread and they ate.

(It seems that the two messengers came to Lot on the same day, or at least during the same time frame, when Avraham and Sarah had received the news about the birth of Yitschaq. The fact that Lot and his people baked unleavened bread, is a fair indication that this may have been a forerunner of what became known later as the Feast of Unleavened Bread)

Gen 21:2 So Sarah conceived and bore Avraham a son in his old age, at the appointed time ("mo’ad") of which Elohim had spoken to him.

Gen 21:8 And the child grew and was weaned, and Avraham made a great feast on the day that Yitschaq was weaned (Most probably as a "double" celebration: In remembrance of Yitschaq’s birth a few years ago AND the observance of a feast that had already existed and was celebrated each year).