1) Choose a difficulty
2) A date will show up.
3) Press the button of the day of the week of that date or press one of the numbers on your keyboard (0 for Sunday, 1 for Monday, ..., 6 for Saturday).
4) A text tells you if your answer is correct or not (and what the correct answer was).
5) A new date will show up.
...

This website is made for people who are trying to learn how to calculate the Day of the Week of a random date within seconds. https://www.wikihow.com/Calculate-the-Day-of-the-Week
https://lifehacker.com/5848651/how-to-quickly-figure-out-the-day-of-the-week-any-date-falls-on
https://blog.artofmemory.com/how-to-calculate-the-day-of-the-week-4203.html
https://en.wikipedia.org/wiki/Determination_of_the_day_of_the_week
http://www.recordholders.org/en/records/dates.html