Why did Galyus go out early and late?
Rashi (citing Sotah 42b): It was to be Mevatel them from Keri'as Shma. 1
Radak: It was to scare them
Be'er Sheva (42b): Perhaps he knew that they cannot overcome Yisrael due to the merit of Keri'as Shma. Rav Elyashiv (42b) - it was not yet the time to fight, so they were not exempt. Amidst great fear, they did not recite Shma now. (It distracted them too much to have intent, just like a Chasan is exempt (Brachos 2:5). - PF)
What is the significance of 40 days?
Sotah 42b: It corresponds to the 40 days in which Torah was given. Radak - Yisrael could overpower him only after 40 days, for at the end of 40 days Moshe descended from Har Sinai and they received the Torah.
Radak citing Rus Rabah 2:20: He was able to do so in the merit of Orpah, who accompanied Na'ami 40 steps. Herpah, his mother (see Shmuel 2:21:18, 19), was Orpah.
Malbim (17): After 40 days, the rations of Yishai's sons ran out, so he sent David with more.
