Fruit is essential for health. You do not need a lot of it, but you should have a daily dose of varied fruit. Best have whole fruit. Drinking fruit juices instead of eating whole fruit may increase your intake of fruit sugars.
Fruit is best bought fresh, according to season, from local sources, like the fresh market. This provides a quality choice and a low price. Out-of-season fruit usually comes from far away, with long transport distance and high cost. So, first thing, check the local fruit season.
Some fruits grow year round. Sweet bananas grow everywhere, all the time. This overview is for Thailand, there certainly are regional differences, nature is not always exact, so these are rough estimates. Also, some fruit may still originate from southern or northern neighbors:
All year: Banana (sweet banana/kluai nam wa), coconut (ma-phrao-on), guava (fa-rang), jackfruit (ka-nun) possibly not year round in northeast and north.
January
Currrent: Grape (a-ngune), lime (manao), green tangerine (som khiaw-waan), papaya (ma-la-ko), rose-apple (chom-pu), strawberry, tamarind (ma-kham), zalacca (la-kham)
Starting: Watermelon (taeng-mo), mandarin
Ending: Gooseberry (ma-yom)
February
Current: Grape (a-ngune), green tangerine (som khiaw-waan), lime (manao), mandarin, papaya (ma-la-ko), rose-apple (chom-pu), watermelon (taeng-mo), strawberry, tamarind (ma-kham), zalacca (la-kham)
Starting:
Ending: Green tangerine (som khiaw-waan)
March
Current: Grape (a-ngune), papaya (ma-la-ko), rose-apple (chom-pu), strawberry, tamarind (ma-kham), zalacca (la-kham)
Starting: Mango (ma-muang)
Ending: Lime (manao), watermelon (taeng-mo)
April
Current: Mango (ma-muang), papaya (ma-la-ko).
Starting: Lychee (lin-chi), pineapple (sapparot), rambeh /burmese grape (ma-fai)
Ending: Grape (a-ngune), rose-apple (chom-pu), strawberry
April is usually still strong dry/hot season. Lowest number of local fruit varieties. Prices of limes April/May sky-high. April Songkran (Thai New Year) has northern traditional recipe's for Jackfruit (ka-nun). Possibly also OK to get some imported apples and oranges at the super market.
May
Current: Lychee (lin-chi), mango (ma-muang), papaya (ma-la-ko), pineapple (sapparot)
Starting: Mangosteen (mang-khud), rambutan (ngo)
Ending: Rambeh /burmese grape (ma-fai)
June
Current: longan, mango (ma-muang), mangosteen (mang-khud), papaya (ma-la-ko), rambutan (ngo)
Starting: Custard apple (noi-naa), durian (thu-rian), langkong (lamyai)
Ending: Lychee (lin-chi), pinapple (sapparot)
July
Current: Custard apple (noi-naa), durian (thu-rian), langkong (lam-yai), longan, mango (ma-muang), mangosteen (mang-khud), papaya (ma-la-ko), rambutan (ngo)
Starting: Dragon fruit (geow-man-gon), lime (manao), (rose-apple (chom-pu)
Ending:
August
Current: Custard apple (noi-naa), dragon fruit (geow-man-gon), langkong (lam-yai), longan, lime (manao), mangosteen (mang-khud), papaya (ma-la-ko), rambutan (ngo), rose-apple (chom-pu)
Starting: Pomelo (som-oo)
Ending: Durian (thu-rian), mango (ma-muang)
September
Current: Dragon fruit (geow-man-gon), langkong (lam-yai), lime (manao), pomelo (som-oo), rose-apple (chom-pu)
Starting: Grape (a-ngune), longkong, green tangerine (som khiaw-waan), sapodilla [also: sawo] (la-moot)
Ending: Custard apple (noi-naa), mango (ma-muang), mangosteen (mang-khud), rambutan (ngo)
October
Current: Grape (a-ngune), dragon fruit (geow-man-gon), lime (manao), green tangerine (som khiaw-waan), papaya (ma-la-ko), pomelo (som-oo), rose-apple (chom-pu), sapodilla [also: sawo] (la-moot)
Starting: Gooseberry (ma-yom), star-fruit (ma-feung)
Ending: Langkong (lamyai), longkong
November
Current: gooseberry (ma-yom), grape (a-ngune), lime (manao), green tangerine (som khiaw-waan), papaya (ma-la-ko), rose-apple (chom-pu), sapodilla [also: sawo] (la-moot), star fruit (ma-feung)
Starting:
Ending: Dragon fruit (geow-man-gon), pomelo (som-oo)
December
Current: Gooseberry (ma-yom), grape (a-ngune), lime (manao), green tangerine (som khiaw-waan), papaya (ma-la-ko), rose-apple (chom-pu), star fruit (ma-feung)
Starting: Strawberry, tamarind (ma-kham), zalacca (la-kham)
Ending: Sapodilla [also: sawo] (la-moot)
Double-check: mandarin, orange (som/som khiau waan), pomme-granate (tab-tim),
Note, while above list may serve as indication, we provide no guarantee that it is correct.