The City of Chicago was incorporated on March 4, 1837, so Monday will be Chicago's birthday.
And in celebration, most employees of the City of Chicago (and of the surrounding County of Cook) will get the day off. Technically, their holiday won't be for Chicago's birthday but, rather, in honor of Casimir Pulaski Day. In Illinois, the first Monday in March is designated Casimir Pulaski Day in honor of the American Revolutionary War hero (born in Poland on March 6, 1745) who is regarded as the "father of American cavalry."
Pulaski has no connection with Illinois whatsoever, but Chicago was at one time the largest Polish city in the world, larger even than Warsaw by some counts, so you might be forgiven if you think that Chicago, and by extension, Illinois, honor Pulaski because of our large Polish population. This is at least partially correct.
Monday is also March Forth Day -- March forth on March 4th, get it? -- a day to 'march forth' and do something positive to achieve your goals or realize your dreams.
March 4 is also Hug a GI Day (be sure to get their permission first), National Pound Cake Day, Learn What Your Name Means Day (also referred to as Fun Facts About Names Day, and Courageous Follower Day (which sounds just a little contradictory to us).
March 4 used to be Inauguration Day, and Congress met for the first time on March 4, 1789.