Session venues in Shrewsbury have a habit of moving around, but possible places (pubs) to check would be The Wheatsheaf (High Street), The Canal Tavern (Castlefields), The Globe (Coleham Head), The Swan (Frankwell), The Loggerheads (Church Street), The Dog and Pheasant (Castlefields). I'm afraid I couldn't even begin to suggest which night would be worth checking which pub.