I think this is one of those edge cases where, because the start date is the end of the month, no one fix is going to make people happy. Change it around and others would say “My December trends start in November?!”
To play devil’s avocado for a moment, the simplest solution would be to set trends to start on the 1st of the month instead, which is only a day or two different. Is there really anything that makes that not a viable solution?
I’d say this, you don’t need to label it as a month. Just do what Summary did and list the dates. I assume it’s only labelled because Trends used to only work with calendar months.
To answer the above question, it’s absolutely necessary that trends can use custom periods, otherwise it’s useless for how a lot of people use it. People want to see payday to payday spend, not random date to random date spend.
If a month had to be shown, then I’d rather it say December in this case, like OP - trends shows all the spending and transfers in a month, most of which happen in December in this case. This is like saying “in November you spent this much on books”, if that spend was actually in December (which is much more likely) then that’s very confusing. I’ve been caught out by this repeatedly i.e. I keep going to the wrong trends month to look for stuff. But just removing the month is definitely a better solution because your payday could be mod month in which case the label is always gonna be quite inaccurate.