Hi Sara,
I also live in Italy, in downtown Milano, where temps fall just slightly below 0 Celsius only for a few days in January, and I keep my three chinese elms outside all year long without any problems.
So, depending on where you are in Italy, I would suggest to put yours outside as soon as temps go steadily above 5 degrees.
And yes, I fully agree with Leatherback, repot it in good, well draining substrate in late winter - early spring (if you are in northern Italy that would be around Mid-March).
I use akadama for all my trees.