@Fil Albuquerque,
I visited Venice in early March, and New York in early April. No problemo.
Would love to go back to both places at those same times (send money!!).
I tend to walk cities - have a big interest in urban design - and have usually visited in early spring or mid-fall. There can be tourist clumps but they are avoidable. I was in Lucca in late April and saw some clumps, but they were easy to skirt around.
Also, I'm odd - night blind, so I don't waltz around alone at night. Thus I'm an early riser and walker; even with a companion, daylight is way easier. Been in the roman forum almost entirely alone: my husband with me, and two japanese guys a hundred feet away (also in early March). Saw no one else except the ticket guy.
Naturally I'd love to check out London (have only stopped at Heathrow) and (aren't you Portuguese?), Portugal in its entirety.