I think for as small as the country is, it has a lot of climate variations. So probably the best advice is to pack layers.

We were in Emilia-Romagna (Bologna, Ravenna, Rimini area) in late June and it was just brutally hot (over 90F in the daytime - sorry I still think in Fahrenheit even when I'm in Europe, so that's how I remember it) But the next night in Milan was very comfortable. There was still snow on the Alps as we continued to travel north.

Venice, Trento and Trieste we did in April, and the weather was nice - a bit of rain, daytime highs mostly in the 50s. As you'd expect, Venice is very damp. I've heard it's actually not all that pleasant in full summer, and I could definitely see how that would be.

And yes - I'm jealous too - eagerly looking forward to my next trip!