Searching google reveals both pomeriggio del lunedi and pomeriggio di lunedi are possible although I wonder if they mean the same or are as common.
I used Pomeriggio di lunedì and it says it is wrong, but to translate it directly did not seem right.
Why is it now important to use the accented i? It wasn't like that in earlier lessons!
Could you say "Lunedi sera"? I know a fair few Italians who use "sera" for what a brit would call afternoon.
No imperative verb here -- in fact, no verb at all! You could make an imperative sentence, like "Call me on Monday afternoon."