Marsala is at its best in late spring and early autumn. April and May give long warm days without the August furnace; September brings the vendemmia (grape harvest) and the salt harvest in the lagoon, which is when the pans turn brilliant pink and the windmills are most photogenic.
July and August are reliably hot — well into the mid-thirties — and the town fills with Italian holidaymakers. Winters are mild and grey, with few tourists, and the surrounding wine country is closed up tight; you will have the streets to yourself but a number of restaurants will be shut.