Frankly, now.
If you mean weather, Cuba enjoys a tropical climate year-round. High season is also dry season, and Havana’s limited hotels can quickly fill up in this November-April window, when daytime temperatures average in the low 80s. It can get even chilly at times at night, especially in January and February.
May to October, the weather is wetter and hotter, but often still pleasant. Hotel rates tend to be lower then, but the influx of new visitors seems to be driving prices up and availability down all year.