I. Prepositions of time are omitted before these words: last, next, this, that, some, every, all, this, these, one, another, the other.
• We met last month.
• We meet every day.
II. "At", "on", "in" are optional in the following cases (but only these three prepositions):

1. When the phrase refers to times at more than one removed from the present:
• (On) the day before yesterday.
• (In) the January before last.
2. In postmodified phrases containing "the" the preposition is optional in American English:
• We met the day of the conference.
• We met the spring of 1983.
3. However: We met in the spring. (cannot be omitted because there is nothing after the prepositional phrase.)
4. In phrases which identify a time before or after a given time in the past or future:
• (In) the previous spring.(the spring before the time in question)
• (At/on) the following weekend.
• (On)the next day.