It's January and dark and cold, so most people suffer a touch of the January blues during this month.
Can you inject some fake summer moments into their day to brighten things up?
We used to have office picnics on a Friday lunchtime occasionally with everyone contributing and reminiscing about previous summer holidays or where we'd live or what we'd do if we won the lottery etc. Several people were pretty good cooks so it made a change from boring cheese sarnies. Even the miserable types seemed to enjoy it.
Absolutely no work talk was allowed!
However, if morale is unusually low, you need to work out the root cause and work to resolve that.