I remember in 2010, when SDC was celebrating its 50th, they were doing special promotions for those turning 50 that year or who were celebrating a 50th wedding anniversary. My in-laws celebrated their 50th anniversary that year, and they both got into SDC for free, along with some other goodies.
For the 50th celebration I think you had to either show your drivers license (for birthday proof) or your marriage certificate... my mom-in-law had to do some digging to find it! haha
I like what they have done this year with the buttons/ribbons and special perks, if you are there the day of (or maybe close to it).