This weekend, the Bing Crosby Theater is offering up two holiday-themed vocalist showcases just in time for Christmas: “Harmony for the Holidays,” which stars tenor Jonathan Mancheni and soprano Isabella Ivy, and “Christmas at the Bing,” a night of music hosted by Douglas Webster and featuring performances from several vocalists, including locals Krista Kubicek and Max Mendez. “Harmony” is a sort of two-hander featuring Mancheni and Ivy, trained opera singers who have been dating for a little over a year. The program is a mixture of standard Christmas classics (“O Come All Ye Faithful,” “White Christmas,” “Baby, It’s Cold Outside”) and less traditional picks, such as “Let’s Drink from the Joyful Cup” from Giuseppe Verdi’s opera “La Traviata” and “Lonely House” from the 1946 musical “Street Scene.”