Question: how do you write 12 noon in a letter?
Answer: Lowercase a.m. and p.m. and always use periods.
Lowercase noon and midnight.
Do not use 12 noon or 12 midnight (redundant). Use noon or midnight.
Do not use 12 p.m. or 12 a.m. Use noon or midnight.
Do not use 8 a.m. in the morning (redundant) Use 8 a.m.
Do not use o'clock with a.m. or p.m.