The first 8 slides each contain a block of 50 years, with each year being assigned a letter. The next 168 slides are calendar months, each identified with a month name and a letter.
To find a date, simply look up the letter for the year, then look up the month you want that is followed by that letter.
For example, let's say you want to find out on what day of the week June 13, 2067 falls. First, look up 2067, which has the letter G next to it. Next, look up "June G", and you'll see that June 13, 2067 falls on a Monday!
Need a year that isn't listed? The calendar repeats exactly every 400 years, so as long as your year is after 1752 (when the Gregorian calendar was adopted), you simply have to adjust your date in multiples of 400. When was July 4, 1776? That's 400 years away from 2176, which uses calendar I. Looking at July I, we see July 4, 1776 fell on a Thursday!