Apart from being the hottest month of the year (around 70 degrees), Italy teems with visitors. Avoid August when the entire country goes on holiday and many hotels, restaurants, and shops close.
Off Season: November to March
Avoid the low season when many hotels and restaurants take a month or two off and the temperature drops dramatically, averaging around 40-degrees.
Sweet Spot: April to May; late-September to October
Early spring and fall, when the weather is still warm (in the 60s) and the crowds are thin, are great times to whiz around Milan's shops, museums, and galleries.