...by my calendar the 4th is a Monday

Well, yes, but since the holiday is on a Monday this year, for most USns it's a three-day weekend. Except for the folks who get sick tomorrow (Friday), for whom it'll be a four-day weekend.

Recall that a while back they tried to make Independence Day into another Monday holiday so it would be a perennial three-day weekend, but there was grassroots resistance. Making it not the Fourth of July but rather the Closest-Monday-in-July never caught on (long weekend notwithstanding), and the practice was rescinded after just a few years.