16/05/16 What do “a.m., A.M.”; “p.m., P.M.” stand for and how are they used?

e-legal lesson

It does not matter whether capitals or lowercase letters are used, as long as a document is consistent throughout. The lowercase letters are now more common.
The phrases for which these abbreviations stand are ante meridiem and post meridiem, not meridian.
Periods are preferred in these abbreviations.