There sometimes comes situations when you need to know the weekday of some particular date. If it's close enough to the current date, or some other date you know the weekday it's quite simple to count the weekday. But when you don't have any reference point it becomes a trickier.
The doomsday algorithm solves that problem. It's a simple mathematical formula you can use to calculate the weekday of any date. In addition to knowing the formula you also need to memorize few set of rules to assign certain numbers based on the century the date is on and a number for each month.
There is also an exception to the month numbers of January and February depending whether the year is a leap year or not. So you also need to know how to figure out that.
It's not rocket science, but you do need to be able to memorize all those things, be able to hold a few numbers in mind and do some relative simple math in your head (of course you can always also use paper or your phone to track those numbers).