Can I only buy it in season?
It is listed as Harvest listed April–June. The harvest period can differ by growing area and variety. The calendar does not guarantee that the fruit is on sale on any given day.
Lychee · ลิ้นจี่
If you feel like a fragrant, juicy fruit, try lychee. Peel the red skin and the white flesh appears, and one bite gives you sweetness and aroma together. If you have had it in drinks or desserts, you may also want to taste the fresh fruit in season.

Harvest listed April–June
Timing differs by growing area and variety
Samut Songkhram Kom lychee: harvest listed April–May
Use the main season, or the harvest period of a regional fruit, as a guide. Actual harvests can vary with the growing area, the variety and the weather.
The white flesh is sweet, and depending on the variety you may notice some tartness. You peel it and take out the seed in the middle before eating.
Lychee varieties such as Hong Huay and Chakkraphat are known in Thailand. Samut Songkhram's Kom lychee is also introduced as a regional GI fruit, which makes it a good way to compare fruit from different growing areas.
It is listed as Harvest listed April–June. The harvest period can differ by growing area and variety. The calendar does not guarantee that the fruit is on sale on any given day.
Look separately at the unit when whole fruit is sold by the kg and when it is sold in a fixed tray. Lychee drinks on a menu and fresh fruit on sale may not share the same harvest period.
If you want to read more about the food and the regions behind it, these sources are a good next step. Ingredients and cooking methods can differ from one place to the next.
Sharing image credit: B.navez · CC BY-SA 3.0