In 1956, Rolex started selling the Day-Date, the very first watch to show the day of the week fully spelled out on the dial. It was essentially a souped-up version of the Datejust, available only in precious metals. Only a handful were ever made in stainless steel, and these are about as collectable as Rolex watches can get.
A few years later, in the 1960s, Rolex's more affordable sister brand Tudor added the Date+Day to its line-up as the analog to the already famous Day-Date. Significantly, this watch was available only in stainless steel and two-tone gold-and-steel models, clearly differentiating it from the all-gold Rolexes. Various Date+Date styles were made over the years, with different case sizes, different bracelets, and different dial and bezel combinations, while the Rolex Day-Date has remained the more conservative model. In today's vintage market, you can expect to pay at least three times or four times as much for a Rolex Day-Date in solid gold as you will shell out for a steel Date+Day.
Getting the Tudor for a fraction of the price of the Rolex is appealing, especially since many of the components were made in the same factories, and the design is so similar. It's a flat-out great watch.