What month do lychees ripen and hit the market? Are there lychees in December?

Generally, the lychee ripening season is from June to July. Some varieties are different, with some ripening earlier and some later, but they generally hit the market in June and July.

Feizixiao: a lychee variety of relatively good quality, ripening in early to mid-June.
Zhuangyuanhong: an early-ripening variety, ripening in mid to late May, with medium quality.
Nuomici: the finest of lychees, ripening and hitting the market in mid to late June.
Guiwei: one of the finest lychee varieties, ripening in mid to late June.
Heiye: a widely cultivated mid-season good variety in Guangdong, ripening in late June.
Huaizhi: a major cultivated variety in Guangdong, with medium quality. Older trees give high yields, and its output currently ranks first among all varieties. It ripens from late June to early July.
Gualv: an exceptionally fine variety, with especially crisp and crunchy flesh, ripening from late June to early July.
Agricultural techniques are getting better and better now, so lychees are available in December, but they are generally rarely sold.