When it falls
Holi falls on Purnima (the full-moon day) of the lunar month of Phalgun, usually in March, marking the arrival of spring — see our article on why Hindu festival dates change every year for how this is calculated each year.
Which deity is worshipped
Holi centres on Krishna, particularly his youth in Vrindavan and Mathura, though the eve of Holi (Holika Dahan) is also a moment of devotion to Vishnu, whose devotee Prahlada is at the heart of that night's story.
Why it's celebrated
Holi carries two distinct traditions that arrived at the same celebration. The first is the story of Holika and Prahlada: the demon king Hiranyakashipu's sister Holika, who had a boon making her immune to fire, tried to burn young Prahlada — a devoted follower of Vishnu — by carrying him into flames while she sat unharmed. The boon failed her because she misused it, Prahlada emerged unhurt through his devotion, and Holika burned instead — read the full story of Prahlada and Narasimha for the wider context. The second, more playful tradition traces the colour-throwing to Krishna's youth in Vrindavan and Mathura, where he is said to have playfully smeared colour on Radha and the gopis, partly because he was self-conscious about his own dark complexion next to Radha's fair one.
How it's celebrated
On the eve of Holi, communities gather around a bonfire for Holika Dahan, offering prayers and sometimes roasting grains in the fire. The following morning is Rangwali Holi — the main event — when people step outside with dry powder (gulal) and water, often mixed with water guns (pichkaris) or balloons, and colour everyone they meet regardless of social status, often accompanied by sweets like gujiya and drinks like thandai. The festival looks different across India: Mathura and Vrindavan hold some of the most elaborate celebrations, including Lathmar Holi, where women playfully beat men with sticks in a re-enactment of local legend; Shantiniketan in Bengal celebrates it as Basanta Utsav with cultural performances rather than just colour-throwing.