Tristram explains that he's going overboard on detail because he just knows that his readers are going to want to know every last tidbit about his life.
So it's important to know that his father is very regular. So regular, in fact, that he winds the clock and has sex with his wife at the same time, on the first Sunday of every month. (His poor wife has some inconvenient associations with clocks.)
Tristram sees a little note in his father's diary explaining that he'd been away from Lady Day until the second week in May, so he could only have wound the clock at the beginning of March.