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What the Day is an online platform designed to help users master the Doomsday algorithm, a mental method for determining the day of the week for any given date, past or future. The service provides a variety of interactive learning tools and challenges that focus on developing speed and accuracy in date-to-weekday conversion, aiming to enhance mental agility and make the process engaging.

The platform offers a step-by-step course covering foundational concepts such as doomsdays, anchor days, and leap years. Users can engage with practice games that drill sub-skills through fast, focused mini-games, and test their abilities by finding the weekday for random or historical dates. A daily challenge presents the same five dates to all participants every 24 hours, fostering a sense of community and competition.

For those interested in competitive play, What the Day features an Arena mode where users compete head-to-head in ranked battles, racing to provide the correct answer fastest. Performance in the Arena affects a user’s rating, with a tier system that allows for progression from Bronze to the Doomsday tier. Additional resources include long-form guides, memory techniques, and structured practice plans to support different learning preferences.

What the Day is accessible through the web and does not require installation. The platform is presented as a side project by Jesper Kiledal, who retains all rights. The tool is suitable for anyone interested in developing mental math skills or learning the Doomsday algorithm for practical or recreational purposes.