Kind of a goof, that got lost in the shuffle
In watching the movie, one can get caught up in the plot and not realize that Fred and Doris started seeing each other, became serious, and were ready to move in together in the space of time between THANKSGIVING and CHRISTMAS.
Even when leaving Long Island to go back to NYC, Fred had to ask/mention to Doris that he could give them a ride. Wouldn't it be implied that a serious couple would already be traveling together.
Then, at the house, that Suzy wanted as her "present", Fred gives Doris what looks to be
their first serious kiss.
I love the movie, but it's a bit of a stretch of artistic license.