The rule underneath all of them
Where winter is cold enough to stop growth, the plant goes dormant and the first growth after dormancy is slow, concentrated and prized. Where it never gets cold enough, there is no dormancy, no first flush and no premium attached to earliness. Everything below follows from that one fact. The calendars differ because the climates differ, and the vocabulary differs because each region named its own.
India and Nepal: flushes
FIRST FLUSH runs roughly late February to April depending on the district and the year. In Darjeeling it produces the light, brisk, floral style the district is known for internationally, and it commands the highest prices. SECOND FLUSH runs roughly May to June and gives a fuller, fruitier, more muscatel character. In Darjeeling this is where the leafhopper effect appears, and many drinkers prefer it to first flush. RAINS or MONSOON FLUSH follows through the summer and is high-yield, thin and mostly destined for blending. AUTUMNAL flush comes in October and November — softer, rounder, less prized than the first two and cheaper than both. Assam runs a comparable pattern with a later start and its own second-flush peak. Nepal's calendar tracks Darjeeling's closely because the terrain is continuous.
China: a solar term, not a month
TeaHQ's own reading of well-attested general knowledge, with no single source behind it.
The Chinese premium is attached to picking BEFORE a specific date in the traditional solar calendar rather than to a flush number. MINGQIAN — pre-Qingming — means picked before the Qingming festival, which falls around 4–6 April. Mingqian tea is the earliest and most expensive material of the year for the famous green teas, and the term appears on packaging as a price claim. YUQIAN — pre-Guyu — means picked before Grain Rain around 20 April, and is the next band down: still spring tea, more of it, less expensive. After that, spring tea generally, then summer and autumn pickings which for green tea are much less valued. Oolong districts run different calendars again, and several pick a prized autumn crop; Wuyi and Anxi are not on the green-tea premium schedule at all. The important caution: Qingming is a date in the north, and the premium is meaningful where the season actually starts around then. In warmer southern provinces the crop may be well under way by early April, which is why "mingqian" travels less well as a quality signal than its price suggests.
Japan: numbered harvests
ICHIBANCHA is the first harvest, roughly late April into May depending on latitude — Kagoshima in the warm south starts weeks before Uji. SHINCHA means "new tea" and is ichibancha sold immediately, fresh, as a seasonal event rather than a separate harvest. It is the same leaf sold on its youth. NIBANCHA is the second harvest in June, SANBANCHA the third; both are cheaper and coarser, and much nibancha and sanbancha goes into bancha, hojicha and bottled tea. Tencha for matcha and gyokuro come overwhelmingly from ichibancha, because those products cannot carry coarser material. Japanese packaging frequently gives a harvest date and a packing date, and the difference between them is worth reading.
Taiwan, Africa and the year-round producers
TAIWAN picks several times a year and, unusually, prizes both spring and WINTER crops for high-mountain oolong — the cold-season pick is a premium product rather than an afterthought. KENYA and the equatorial African producers pick year-round on a seven-to-fourteen-day round, with quality varying by rainfall rather than by season in the flush sense. There is no first flush and the concept does not apply. SRI LANKA also picks year-round, with quality peaks tied to the two monsoons hitting different sides of the island at different times — so the best months differ between the western and eastern districts. ARGENTINA, being in the southern hemisphere, runs its season from roughly October to April, which inverts every date above.
What to do with this as a buyer
Match the vocabulary to the origin before reading it as quality. "First flush" on a Chinese tea is a translation somebody chose; "mingqian" on an Indian tea would be meaningless. Where earliness carries a premium — Darjeeling first flush, mingqian greens, shincha — freshness matters correspondingly more, because you are buying a seasonal character that fades. Where it does not, a harvest date is still useful and the urgency is lower. And remember the calendar moves. These are typical windows, not fixed dates, and a cold spring shifts everything by weeks.