I originally used LOOKUP2 because my first rough code this morning used the last day of each month cumulative, which means as you get towards the end of the year the DAY spills out of a byte variable, before I changed the code to the version I actually posted, and forgot to change it back.

You can of course save yourself another 14 words of codespace by not caring that your code runs past year 2100 and removing the century leap-year correction. As if we're going to lose sleep over it...