Easter is a movable feast, set on the first Sunday after the first full moon on or after the twenty first of March, and the arithmetic of that rule lands it in April more often than in March, which makes April the usual month of the central Christian festival, with Holy Week, Good Friday and the fifty days of Eastertide flowing from it. Passover, whose date in the month of Nisan the Easter computation was built to track, most often overlaps the same weeks, so the two festivals of liberation and resurrection usually share the April moon.
The middle of the month carries a band of solar new year festivals that are fixed rather than movable. Vaisakhi, on the thirteenth or fourteenth of April, is both a harvest new year in Punjab and the anniversary of the founding of the Khalsa in 1699, when Guru Gobind Singh called the first five initiates before the assembly at Anandpur, which makes it the most consequential single date in the Sikh year. The same solar reckoning gives Southeast Asia its new year at the same moment: Songkran in Thailand, from the thirteenth to the fifteenth of April, washes Buddha images and elders' hands and has grown into a national water festival, with parallel celebrations in Myanmar, Cambodia and Laos, and Tamil, Bengali, Nepali and Sinhala new years falling in the same few days.
Modern Pagan practice keeps no major station of the Wheel of the Year in April, but the month closes on the eve of Beltane, and Walpurgis Night on the thirtieth carries its bonfires into the first of May.
Sources The Oxford Companion to the Year: An Exploration of Calendar Customs and Time-ReckoningBonnie Blackburn and Leofranc Holford-Strevens with The Encyclopaedia of SikhismHarbans Singh (editor-in-chief), Songkran in Thailand, Traditional Thai New Year FestivalUNESCO Intangible Cultural Heritage