Why does it mention the moon?
Radak: Now it discusses what was created on day four. It begins with the moon, even though it is the small Ma'or, because the night precedes the day. On the fourth night, the moon and stars served, and in the daytime, the sun shined.
Malbim: The benefit of the sun (its light) is known. Also the moon is useful - for Mo'adim.
How is the moon for Mo'adim?
Radak: "V'Hayu l'Osos ul'Mo'adim" (Bereishis 1:14). He made the moon vary from Chaser to full and Chaser again, in order to count months 1 . The verse mentions the moon; also the sun is for Mo'adim 2 , like it says in Bereishis. Also the stars, which rule with the moon at night, are included.
Rashi (Bereishis 1:14): Yisrael will be commanded about Mo'adim, and they count them [based on the months, which are] according to the Molad of the moon (when it is in between the earth and sun, so the side facing the earth is totally dark).
Sometimes we add a month to the year to compensate for the difference between a solar year (365.24 days) and a lunar year (12 months, about 354 days - PF).
Why does it say "Shemesh Yada Mevo'o"?
Rashi (from Rosh Hashanah 25a): We infer that the moon does not know its way. (There should be 24 hours before the old moon and the new moon;) sometimes [the time in between] is long (more than 24 hours), and sometimes it is short.
Radak #1: The sun's light is its own. The moon does not 'know' its light, for it itself is black; it merely reflects the sun's light 1 . Mevo'o refers to sunrise and sunset. However, since there is another word for sunrise (Zerichah), usually 'Ba ha'Shemesh' refers to sunset.
Radak #2: When the sun [first] shined on Wednesday morning, it knew that it is Mevo'o (setting) of the moon.
Malbim: The half of the moon facing the sun is always shining; the half facing away is always dark.
