Thank you for your answer. I'm sorry that I misleadingly used the word "perpetual" in my post above. I didn't mean to imply that the Senator Calendar Week is anything like a perpetual calendar or that such a mechanism would be needed to determine whether a year has 52 or 53 weeks.
In fact, the Senator's combination of day, weekday and calendar week is all that is needed. The rule is as follows:
If on Sunday of the 52nd week the day is the 26th or 27th, the next week number is 53. In all other cases the next week number is 1.
Even without that the Senator Calendar Week is truly a great watch.