The offer is only good on Thursday, April 21, 2016, from 11 AM to 3 PM, at participating locations. It's limited to one sub discount per person per order and is good on any of the chain's regular 8" subs (#1 - #6, BLT, and Slims). Apparently, you can stand in line multiple times if you want multiple $1 subs though. The deal is only good for in-store orders.
The chain's 8" subs normally cost around $6 or so.
You can find a list of participating locations on the brand's website here.
Photo via Jimmy John's.