When exactly, was Yitzchak born?
Rashi #1: There are two opinions in the Medrash (Rabah) as to whether he was born after nine months, or after seven. (However, both opinions in Rosh Hashanah 10b agree that he was born on Pesach.)
Rashi #2: Based on the word "Oso" 1 , the Tanchuma explains that he was born exactly one year later (on Pesach).
See Sifsei Chachamim.
What does "la'Mo'ed" mean?
Rashi #1: It means after nine months (the appointed time that a woman gives birth). 1
Rashi #2: It means after seven months (the time to which the angels referred earlier in the Parshah).
Rashi #3: Exactly one year later, when the sun reached the same point on the wall as it had been when the angels came to see Avraham. 2
What is the meaning of 'li'Zekunav"?
Targum Onkelos and Targun Yonasan #1: It means literally 'in his old age'.
Targum Yonasan #2 and Rashi: It is the acronym of 'Ziv Ikunin' (his shape of face), because Yitzchak's face was strikingly similar to that of Avraham.
Why does it need to say that she became pregnant? It suffices to say that she gave birth!
Da'as Zekenim: They are put together to teach that just like she became pregnant without pain, she gave birth without pain. 1
Sotah 12a expounds so about Yocheved. Righteous women are not subject to Chavah's curse.
רש"י: למועד...דמליל יתיה וכו', שרט לו שריטה בכותל וכו': למה נזקק רש"י לדברי התרגום? ולמה הוסיף שקבע לו זמן ע"י שריטה בכותל?
גור אריה: קשה למה כתוב "למועד אשר דיבר אותו", והרי 'אותו' מתייחס לדבר הפעול ולא כלפי מועד הפעולה, [שהרי הוא פעל באותו זמן אבל לא פעל 'את הזמן']? ואין לפרש כאילו כתוב 'אשר דיבר איתו' שהרי התרגום הוא "דמליל יתיה"? לכן פירש רש"י שקבע לו זמן ע"י שריטה בכותל [והוא כן עשה פעולה ביחס למועד הלידה.]