Once you have determined the Doomsday for a specific year, you can use it to find the weekday of any given date in that year. This is the practical application of the Doomsday algorithm, allowing you to calculate the day of the week for any date quickly.
For instance, let's use the year 1966, for which we've calculated the Doomsday as Monday. To find out what weekday a specific date fell on, like July 20, 1966, you first identify the closest Doomsday date. In July, the Doomsday date is the July 11 (which we know is a Monday). July 20 is 9 days after July 11.
To find the weekday of July 20, add 9 days to the Doomsday. Since 7 days make a week, and we're dealing with days of the week, you can reduce 9 by multiples of 7. In this case, 9 minus 7 leaves 2. So, July 20 is 2 days after the Doomsday (Monday). Thus, adding 2 days to Monday brings us to Wednesday. Therefore, July 20, 1966 was a Wednesday.
This method can be applied to any date in the year. It's a matter of finding the nearest Doomsday, calculating the difference in days, and then adjusting for the 7-day week cycle. With practice, this becomes an intuitive and rapid process, making it easy to determine the weekday for historical events, future planning, or just satisfying your curiosity.