Why did he offer only at night?
Malbim: He was afraid to offer during the day due to people of the city (verse 27).
What is "Par ha'Shor"?
Rashi: It is the young bull. A newborn bull is called Shor (Bava Kama 65b). 1
Radak citing Targum Yonasan: This is like Par ben Bakar.
Radak: Par is not Samuch to ha'Shor. It is a big bull, that resembles a Shor. 2
Hashem said to take the first bull, but did not say what to do with it!
Radak (28): He should steal it, lest they offer it to idolatry. 1
Malbim: Gid'on already intended to offer an animal, just he was unsure if he should offer a ram, lamb or kid.+
I.e. Hashem knew that they intended to offer it for idolatry and would have done so if it was available. (PF)
What is "Par Sheni Sheva Shanim"?
Rashi: It is a second bull; it was fattened for seven years 1 to offer it to idolatry. Malbim - this was not of his father, rather, of people of the city. When Midyan began to rule over Yisrael, they designated it and vowed to offer it to idolatry when they will be saved from Midyan.
Rashi (Shmuel 1 15:9): "Sheni" teaches that it had twice as much meat and fat [as a regular bull].
Radak: Par Sheni is the second born from [its mother's] womb.
Radak: Reish Lakish (Temurah 28b) holds that Muktzah (designation for idolatry) is only after seven years.
QUESTIONS ON RASHI
Rashi writes that the bull was fattened for seven years, for idolatry. One may not offer Muktzah (an animal designated for idolatry)!
Rashi, citing Temurah 28b: This was a Hora'as Sha'ah (a special temporary ruling). Hashem permitted eight Isurim that night - he offered outside the Mikdash (in Shilo), at night, he was a Zar, he offered without Klei Shares, he used Kelim used to worship an Asheirah and wood of an Asheirah, and he offered a Muktzeh and Ne'evad (worshipped) animal.
