I am assuming you are storing the day on which a user needs to be sent a reminder in the database. So you can convert the number to a date. For eg. if a user selects 1 from the dropdown, on save you store the current date change day to ‘1’ (the value of the dropdown) This will save the date for a particular month.
Alternatively, you can define the day as a an input to the API that will trigger the email. So the API checks for all the users which have selected 1 as the date, and then send an email to those users. This might be a more effective approach if you aren’t concerned about saving the date in the database but only the day on which an email is to be sent.