The event runs from 11 AM to 7 PM on Friday, March 18, 2016 and Saturday, March 19, 2016.
While generally anything that can hold a Slurpee without leaking is allowed (i.e. kettles and fish bowls are fair game), there are some limitations: you can only bring one cup-like container (per visit anyway) and it must be able to fit upright through a 10-inch diameter hole (they'll have an in-store display to check). Specifically excluded containers include "trashcans or trash bags, kiddy pools, [and] kitchen sinks."
In case you don't want to bring your own, 7-Eleven will be selling collectible 64-oz mason jar-style cups decorated with a "Slurpee 50" celebration logo (the Slurpee turns 50 this year) for $4.99 at participating stores (the price includes a fill-up so the cup itself is only costing you $3.49).
7-Eleven's current featured, limited-time Slurpee flavors include Airheads Xtremes Rainbow Berry and Diet Coke Frost with a Splash of Cherry.
Photo via 7-Eleven.