Day of the week
A seven-position disc advanced once every twenty-four hours
What it is
An aperture, usually at twelve o'clock, showing the day of the week. The best versions spell the day out in full and change at or near midnight. It is a genuine calendar mechanism rather than a printing choice, because something has to advance a seven-position disc once every twenty-four hours.
History
Day-of-week indication is far older than the wristwatch and runs throughout pocket-watch calendar work. On the wrist it arrived through the triple-calendar and perpetual-calendar watches of the 1940s and early 1950s, where day and month sat in twin apertures below twelve: Patek Philippe's reference 1518 of 1941 established that layout, and Rolex's own references 8171 and 6062 used it around 1949 to 1953. The Rolex Day-Date of 1956 is universally credited with being the first wristwatch to spell the day out in full; treat that carefully, since Rolex's own earlier triple calendars carried day apertures. What is documented is Swiss patent CH322341A of 1955, credited to Rolex engineer Marc Huguenin, and the launch of references 6510 and 6511 in 1956. The safe statement is that the Day-Date put a full-word day together with a date window on a self-winding, water-resistant, chronometer-grade watch, and that is what made the display common.
How it works
A day disc or day star with seven positions sits under the dial and is indexed once per revolution of a twenty-four-hour driving wheel, itself geared down from the hour wheel of the motion work. A finger or cam on that wheel engages a tooth on the day star and pushes it forward, while a sprung jumper holds the disc precisely under the window between changes. In a semi-instantaneous system the finger drags the disc round over a period either side of midnight; in an instantaneous system it first tensions a spring and then releases it, so the disc snaps over. Quickset correction is a separate wheel brought into engagement by a crown position or a case-side pusher, working directly on the day star.
Parts required
A day disc or star, a twenty-four-hour driving wheel with an advance finger, a jumper spring and its screw, plus a correction wheel where the day is quickset.
In the catalog
Related
- Date: The most common watch complication; and the most corrected
- Day-date: Two discs, one timing signal, and the movement’s peak torque demand
- Annual calendar: Knows 30- vs. 31-day months; needs one correction per year