Brit chain Pret A Manger had moved into the former Tisserie space at 857 Broadway (at 17th Street), and now comes official word that when it opens for a test run this Friday, it will give away 3,000 free sandwiches starting at 12:15 p.m. — provided you snag one of the flyers that will be given away at the Union Square subway entrances between 8 a.m. and 9:25 a.m. that morning. Pret is calling this the “Guinea Pig Day” for what will be their U.S. flagship (the store features a fresh design and free Wi-Fi), but for better or worse, cuy won’t be served — instead, Pret is sourcing food directly from Greenmarket vendors. The location opens fully on June 22.