A monthly allowance period determines when your £300 monthly deposit allowance resets.  

This starts from the day you open a pot. For example, if you open a pot on 8th July, your £300 monthly deposit allowance will reset on the 8th of each month. If you open a pot on the 29th or later, and that date doesn’t fall in a particular month, your monthly allowance period will reset on the last day of that month.