BTW, February 26, 1900 is actually Monday, but my post is correct because Excel erroneously thinks it’s a Sunday.
In order to make it compatible with Lotus 123 Microsoft had to introduce a bug… tricking Excel into thinking that 2/29/1900 is a valid date. (It’s not, but Lotus 123 thought it was.)
So the “fix” was to make the days of the week correct for March 1, 1900 and later, but they are all off a day for January 1, 1900 - February 28, 1900.