With this plus the Doomsday algorithm, I am invincible.

For people who don’t know, John H. Conway’s doomsday algorithm allows you to compute the day of the week for any date by starting from a set of memorable dates (called “doomsdays”) and working from there. https://en.wikipedia.org/wiki/Doomsday_rule

So for example doomsday for this year is a Saturday, so I know the last day in February is a Saturday (this Saturday in fact) and also that Independence Day, Boxing Day and Halloween are also going to be on Saturday this year, because they are all Doomsdays (on non-leap years).