'on' for days and full dates
on Tuesday
on the 25th of June (FOR DATES ALWAYS USE 'the' and 'of'.....THE 27th OF July) These, however, aren't always written. In British English (which is what I'm referring to here), we write: "22nd March" BUT we SAY 'the 22nd of March'.
In American English this is slightly different.
'in' for months, years and seasons
in August
in 1987
in winter