Melissa Fitzgerald of the Houston Downtown Alliance points out two especially attractive things about the 20th Annual Theater District Open House: ''It's free and everyone is invited,'' she tells us. The Open House, now in its 20th year, is a showcase for the city's nine major performing arts companies (the Alley Theatre, Gexa Energy Broadway at the Hobby Center, Da Camera of Houston, Houston Ballet, Houston Grand Opera, Houston Symphony, Society for the Performing Arts, TUTS and Uniquely Houston®). The day's schedule includes backstage tours; dance and musical performances; meet-and-greets with costumed performers; season subscription specials; and giveaways at the district's venues, including the Alley Theatre, Jones Hall for the Performing Arts and the Wortham Theater Center. Artists set to perform include members of the Alvin Ailey American Dance Theater and the Houston Symphony. The 20th Annual Theater District Open House is presented by Bank of America, with TransCanada as the title sponsor.