Hello.

I'll be grateful for your help.
1)Is it possible to say "next day" instead of "tomorrow", for example: "I'm meeting with him next day."?
2)Is it possible to use "develop" in the meaning "explain", for example:"He will develop the situation later."?
Thank you very much for help.

1) No. You might say, I'm meeting him the next day after the convention. But we don't use "next day" to replace "tomorrow."

2) No.