i do not know, but my educated guess is Monday and Thursday nights. On the Sapphire coastal cruise in May it was Monday and Thursday nights and those nights would work with the current, albeit different, itinerary. On Monday the Sapphire sails early at 1 pm from SF and formal nights usually occur on either a sea day or a day when the ship sails early. The second formal night on a 7 day cruise usually occurs on the second to the last night of the cruise. On the last night of the cruise everyone is busy packing.
Cunard, Sagafjord (Pacific Coastal), Admiral Cruises, Star Dancer (Mexico), HAL old Noordam (Caribbean), NCL Norway (Caribbean), NCL Seaward (Caribbean), NCL Pride of America (Hawaii), Celebrity Century (Caribbean), Celebrity Mercury (Caribbean), Celebrity Constellation (Scandinavia/Russia), Celebrity Infinity (Hawaii), Renaissance R4 (currently sailing as Ocean Princess) (Tahiti), old Crown Princess (Caribbean), Dawn Princess (Alaska), Star Princess (Australia/New Zealand and Mexico), Golden Princess (Mediterranean, Canada/New England, Mediterranean/Transatlantic, and Hawaii), Sapphire Princess (Alaska, California Coastal, Mexican Riviera, Mexico Christmas Cruise, 4-day Coastal Getaway x 2, and Pacific Coastal), new Crown Princess (Bermuda/Caribbean, California Coastal, and Mexico Christmas Cruise), Emerald Princess (Canada/New England)