The holiday (Holi) is not a pan-Indian holiday, as per for Nagaland, J&K; (or the K at least), etc. It is a Hindu holiday, (albeit Diwali has an added caveat as rightly written) a simple link on there would add the caveat that it is not celebrated by the churches and mosques of india. Which holidays with the caveat are secular? Other than new years, where i have already put the added caveat that it is not universalized as per WP:Common sense