. . . for the Christmas party.
I did wonder if this was actually a Christmas party (as the flowers looked a bit like daffodils and although you can buy daffodils in the shops now I don't think you got them before Christmas back in the 1950s) anyhow a quick zoom in resolved my doubts as there are indeed Christmas crackers on the table and those may be the tips of decorations hung very high. The flowers may be plastic, and may be roses and carnations (plastic ones).
Back to the hats, they haven't pulled the crackers yet, so they aren't out of those. There seem to be 3 varieties on show, a fez, a clown hat and something flatter with maybe a little feather (did Robin Hood wear something similar?).
The lady whose photo this was is on the far left, wearing a fez. They all look to be enjoying themselves.