So now that this has been implemented in a sense. I think a thread to discuss how this works and any input from a monzo employee regarding this would be helpful.
So currently you can ‘tag’ your salary to set the custom date. Now one would assume that the reason for doing it like this is that the summary and targets now go salary payment to salary payment, resetting when that payment is detected for roughly the same amount of money (to rule out false positives for expenses and so on). So this should actually not use specific dates at all, however it seems from some users comments that this isn’t the case at all and tagging your salary just sets the date of that transaction as the monthly date.
If the latter is true then surely it would’ve just been easier to allow the user to select a date and not tag a salary transaction. The way this is currently implemented means that people who do not get their salary paid into monzo cannot set a custom date (mine currently shows no transactions when trying to set the date). If monzo are dead set on this implementation of this then perhaps showing any incoming trasaction to set as salary?
I do realise that it is incredibly beneficial for Monzo to get people paying in their salaries and that this is a good tactic to get people to do so. But there are people like me in a situation where I can’t really do that right now. I have a loan with HSBC that paying a certain amount of money in each month to my current account gives me a better interest rate, so moving my salary now would mean that the interest would almost double.
Just my take on this feature in the way it is, just a little bit dissapointed in it after it being my second most wanted feature since getting 
!
Just like to say that the new summary UI in general is really good and such a great improvement upon the spending tab.