Based entirely on the number of postcard stories that mention these events, it appears that Christmas Dances were a common attraction in towns and cities across the US in the early 20th century.
Mrs. Clyde Mercer lived in Berg, a town that (like many agricultural towns on the prairies) no longer exists in North Dakota.
Berg was located in McKenzie County of west-central North Dakota; the Post Office in Berg was closed in 1944.
https://en.wikipedia.org/wiki/McKenzie_County,_North_Dakota
In December of 1911, Mrs. Mercer received a postcard greeting from her sister, Mary.
The face of the postcard bears a lovely bit of postcard art, a boat tied up in an inlet beside a lakeside cottage.
It is a homey scene; the wash seems to be hanging on the line at the close of day.
Mary wrote from a place near Berg – the postcard bears two postmarks indicating receipt of the post, but it is unclear where the postcard was mailed.
The sisters did not live far apart, as Mary encourages Mrs. Mercer and her husband to “come over if you can.”
The dance will be held on Friday night, and two other persons (Em and Lee, possibly) are expected to join the party.
One hopes that Mrs. Mercer and her household were able to attend the festivities with the other guests of Mary, and that all the dancers enjoyed a wonderful Christmas celebration.
