For good weather and festivals, late March
Nepal (Kathmandu Valley) Macchendrath Rath Jatral, 1 month as 100s of devotees tow a massive wooden ‘chariot’ carrying their #1 rain god through the streets. Patan is one of the 3 towns in the fascinating Kathmandu Valley.
Atitlan), Holy week.
Mexico, Chichen Itza , Spring Equinox, 1 day.
(Chiapas state and nationwide) Holy week Palm Sunday to Easter Sunday.
Vietnam, (Huong Tich Mountain range), Huong Tich/Lac Long Quan Festival (Buddhist pilgrimage), Feb-early Apr
(Quoc Hai) Thay Pagoda Festival, Mar/Apr, 7th day of the 3rd lunar month, 1 day. Vietnam Pictures
Burma (Myanmar), (Nationwide) Thingyan Water Festival, Mar/Apr, 3-4 days.
Colombia (Cartagena), Cartagena International Festival of Caribbean Music. Mid-Mar, 5 days.
(San Antero), Colombian National Beauty Contest for Donkeys.
Nepal, Holi, celebrating Hindu myths in a totally wild, colourful, and throw-paint-over-everyone way. Nationwide but especially good in Kathmandu. March 18.
Philippines (Nationwide) Holy week (the week up to Easter), Mar/Apr, famous mostly for bloody flagellation and crucifixions. Body piercers will love it.
Or go for festivals only, not the weather! i.e. this is not the best season
Tibet (beware extreme cold! ):
(Nationwide) New Year. Feb/ March 4 days.
(Lhasa) Great Prayer fest. Feb/March 7 days.
Peru (Puno, Cusco+) Holy week.
Indonesia, Bali, Nyepi (Balinese New Year), a few days.
(Larantuka, Flores), Easter Parade.
Bhutan, Paro and Thimpu festival, in a stunning monastic setting monks in brilliant silks perform traditonal dances. March/April.
Bolivia (Oruro) Devil’s Carnival Feb/March. 8 days.