After more than three decades serving the Cambridge community and visitors with its famously flamboyant flair, UpStairs on the Square is set to close its doors on Dec. 31, co-founders and co-owners Mary-Catherine Deibel and Deborah Hughes announced in an email to subscribers over the weekend. In an interview with The Crimson Monday, Deibel and Hughes elaborated that the building was up for sale by the landlord. Deibel said that the sale of the building presented a logical time to end the long-running business, and she and Hughes decided not to sell it.