One thing the replies above skip: when. Every store runs fests and sales on a fixed calendar, and a demo that lands two weeks before a themed fest your game fits gets seen; the same demo a week after it doesn't. Steam publishes the fest dates on the partner site, itch has the jam calendar, GOG and Epic announce theirs the day they start, and nobody keeps them in one place with the deadlines. I got tired of it and made one, with the official page linked for every date: https://covermygame.com/events?from=itch
Short version for the next six weeks: Next Fest submission closes 28 September, Autumn Sale is 1 to 8 October, Next Fest runs 19 to 26 October, Scream Fest for horror 26 October to 2 November. If you plan a demo, plan it against one of those.