6th April: Chakri Memorial Day is a public holiday in Thailand
13-15th April: Songkran สงกรานต์ is the Thai New Year (celebration dates are fixed, but vary slightly regionally)
25th April: ANZAC Day, a public holiday in Australia and New Zealand. Dawn and morning commemorative services at war memorials and other places in both countries as well as in Gallipoli in Turkey.
27th April: Freedom Day in South Africa. In celebration of South Africa's first non-racial democratic elections in 1994
29th April: Greenery Day (みどりの日 Midori no hi), Japan
30th April: Walpurgis Night, celebrated in Sweden (Valborgsmässoafton), Finland (Vappu) and Germany (Walpurgisnacht)
Buddha's birthday: The Buddha's birth, enlightenment and passing beyond life and death are all celebrated in Cambodia, Laos, Myanmar, Sri Lanka and Thailand, on the 15th day of the 6th month of the lunar calendar - which is usually in April according to the Gregorian calendar.
30th April (or Saturday 29th if 30th falls on a Sunday): Queens Day, Netherlands
Movable dates
The starting day of Golden Week in Japan, an extremely popular time to travel — best avoided as transport and accommodations are often booked solid
Early April: Kanamara Matsuri (金まら祭り), also known as the Iron Penis Festival, in Kawasaki, Japan