what can i do if my new landlord did not accept my rent for being late?

i receive a 3 day notice or quite letter from my land lord on oct on the envelope its stamp on 2nd i don’t remember what date i receive that letter does the stamp mean i received it on that date? anyhow i always pay my rent on the 7th but since this is a new land lord i was never inform that the rent was due on the 1st. i made a money order and sent his rent on the 8th but today he came to my apartment and return the money order and said he couldn’t accept my rent because it was late he asked me to call him later but his not answering my calls does that mean he will order an eviction form on me what can i do if i want to stay in this apartment i’am a single mother and can’t afford the really high rents im a low income mom help please