I've been living under a rock so this might be old news, but McDonald's has been having a special daily deal since the beginning of the year, celebrating the first 18 days of 2018. Today's deal is a Big Mac and medium drink for $16!
Lineup for rest of week is $10 chicken wings/small drink, cheeseburger/small drink, and some kind of strawberry parfait or McFlurry (see here).
And here's the official link from McD's (looks like it updates daily).
Edit: anyone know how to just have simple links instead of expanding ones?