Just seen this BBC article: www.bbc.co.uk/news/uk-42036462 which suggests that because there are 5 "paydays" in December, many families may lose or get a reduction in Universal Credit for the month and need to reapply the month after.
I don't know any more detail than is in the article but thought it important to share as there isn't exactly much time to put money aside if you will lose money!