Plovdiv is a very good place for a one-day trip or a city break from Sofia. If you happen to be in the region on 17th of September, you might also want to stay for the night. Because it’s going to be a long night… of sightseeing and fun. More than 6 hours of cultural output, 68 locations, 3 main programs, numerous exhibitions, concerts, performances, demonstrations and children’s events. More on their website and the whole schedule in a convenient booklet.
A good idea may be to stay in Sofia during the day for the celebrations during the Day of St. Sophia (which as of 1992 is the official day of the city) and then maybe head to Plovdiv in the evening.
By the way, the Bulgarian capital Sofia participated this year in the 7th edition of the European Night of Museums on 14th of May with over 40 other European countries.