Eng asked when to use in and when to use on. That is a good question. They are both prepositions or I prefer to say "position words".

EXAMPLES

I will go to China on Sunday. (specific day)
I will go to China on the weekend. (specific weekend, day not specific)
I will go to China Sunday. (on not used, dropped)
I will go to China at 2:30pm (specific time)
I will go to China in 2005 (month and day not specified)
I will go to China in September. (day not specified)
I will go to China in summer. (day and month not specified)

Is a rule emerging in your mind?

Seems English uses on for specific days/weekend but in for months, years or seasons. At is used for specific clock time.

Foreign teachers often do not have a CLEAR RULE in their minds to tell EFL students because they learn English differently. They learn it by hearing the language as a child. Later in school they learn SOME of the English grammar rules, but many are simply not taught.
Many rules are studied only by linguists, specialists. Many of the foreign ESL/EFL teachers world-wide were not originally specialists in English or linguists. I'm an example of someone who changed fields. Went from biology to ESL teaching in 1996. We still bring valuable language experience to the classroom. My biology taught me a lot of Latin and Greek roots (at the core of English), and much scientific terminology is adopted internationally (also a core part of modern English).