Is it common to omit a preposition (in / on / of) before “the month (year / week /day) when they are used adjectively and adverbially?

Solution 1:

This is a very good question and it made me do some serious digging. Here is what I found:

In the examples you have given, it is fine to use a preposition. There are cases when prepositions must be omitted and when omission is optional:

prepositions of time are omitted before the words: last, next, this, that, some, every (We met last month. We meet every day.)
"at", "on", "in" are optional in some cases (but only these three prepositions).
1. when the phrase refers to times at more than one remove 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. However: We met in the spring. (can not be omitted because there is nothing after the prepositional phrase.)
3. 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.