I'm surprised noone has chimed in the real answer. It is exactly the same movement, with the difference that one has a day and a date (2836). The 2824 only has the date.
By day, I mean the 3 character version: MON TUE WED, THU, FRI, SAT, SUN sometimes available in different languages.
It is simply a few extra parts and another disc--nothing else is different. Because they're the same movement design, watch companies purchase whatever is on sale. If they don't need the day of the week, they can take the disc out or leave it in--either way it's all the same.
The disc mecanism for the day adds a fraction of a millimeter in thickness. There is another variation which is the 2834 that has the full day spelled out (like a Rolex Day-Date)--again same movement design, but that one has a larger diameter module on the dial side (day of the week is a disc on the outside of the date wheel). yet another version is the 2846 but the difference there is the escapement and less jewels (and a slower beat of 21600).
You can probably still read up on these at www.eta.ch