It’s a form of perpetual calendar, of ecclesiastical origin. See the wiki article on “Dominical Letter.” Its value is that you can easily determine the day of the week that a particular date will fall upon.
[fun answer, not a real answer]Just to be safe, I think you should stay in hiding for a week extra or so, just to be sure.
There was an episode of the 1950’s-era Superman (George Reeves) TV show that focused on this. A fugitive had entombed himself into a crypt, along with all the provisions he might need to live comfortably for some number of years. Everyone knew he was there, but the crypt was built of some super-material that nobody could bust open. (He had a vial of some kind of super-acid to burn through it when he was ready to come out.) The plan was to stay there until some statute of limitations ran out. Superman tried to break in to get him, but failed.
So the authorities just tricked him. They cranked up the voltage of the electricity serving the crypt (one would think, then, that they could just have shut off the electricity altogether) thus causing his clock to run a little fast, whereupon he thought the time was up when he still had a day to go. So he came out a day early and got taken away, and everybody else lived happily ever after.
So don’t play the game of trying to cut it as close as you can.
[/fun answer, not a real answer]