The first year Christopher and I were dating, he wanted to take me out for dinner on my birthday. My favourite restaurant was not plausible because it's not local (or overly accessible by transit, especially on a weeknight after work) and, at that time, he didn't have a vehicle. So he told me he'd take me any place I wanted in our city/region... And I picked McDonald's. My birthday and McHappy Day coincided that year, and I thought it would be cool if the money he was going to spend on my birthday dinner would help support Ronald McDonald House. Punning on "McHappy Day," one of us called it a McHappy Date, and it became an annual thing (though it's not usually my birthday dinner). When we started doing it there were only certain menu items that qualified, but now it's just a portion of all sales so this year we ordered whatever we wanted.
It was very convenient that Have a Cola Day from my holiday calendar fell on a day when we were planning to have fast food anyway-- both of us did have cola as part of our McHappy Date, though mine was diet!