March - April: Spring time is beautiful in Nanjing, nice weather, mellow temperature, very comfortable. There are flowers everywhere.
September - November: Autumn is also a great period in Nanjing, especially when you visit the mountains and park
Worst time to visit Nanjing:
Mid June - August: Starting from mid or end of June starts the rain season in Nanjing where it rains continuously for days. It normally ends somewhere in July, however after that it gets super super hot. temperature could go up to 40 degrees with very strong sun
Winter time is tricky as it's very cold with poor air quality, it also snows often. Temperature is around 0-10 degrees BUT with the humidity you feel much colder
However January is the Plume Blossom season in Nanjing, even when it snows it's stunning