Okay, so wasn't aware of this national day... Anyway Pizza Express just posted on their facebook page - So it might be worth checking your local opening times and try to be their first. If you're anything like me, you're already there :p
Just quote "National Margarita Day" and be one of the first 10 people at your local annnnnd it's yours.
Just so save some people a little time and poss dissapoinment
"Here are a few places you won’t be able to use it: PizzaExpress Ashford McArthur Glen, Brent Cross, Manchester - King Street, Manchester - Trafford Centre, O2 North Greenwich, Olympia, White City – Westfield or at live music rooms"
Thanks to Emribena for this