Baltimore's best and most distinctive architecture gets showcased as part of this worldwide effort to point out just what builders and designers can do with brick, concrete, wood and whatever other materials they can find. More than 60 sites — some old (including the 1814 Peale Museum building, above), some newer, some that defy easy description — will be open for visits, tours and encounters with architects eager to talk about what you're seeing. The Maryland Historical Society, 201 W. Monument St., will serve as the day's hub (admission is free); there you can pick up a map and brochure. Then spend the day visiting homes, commercial buildings and churches dating back to the 18th century. Plus lectures and other presentations at some sites. Prepare to be impressed, and educated. 10 a.m.-4 p.m. Saturday. Free. doorsopenbaltimore.org.