UK Landlord Forum / Life

My tenant moved in on April 17th 2013 and was on a 6 month fixed AST which ended on October 16th 2013. Thereafter she has been on a rolling periodic tenancy.

She paid approx 2 weeks rent at the start of the tenancy to pay for the remainder of April and then we changed the rent date to the 28th of every month to accommodate her payday.

Due to certain issues, I now want the property back so have decided to issue a Section 21. However I am confused as to which date I need to put on-27th of the month (day before next payment date) or the 16th of the month (taking into account tenancy end date)

Any advice would be much appreciated!
Thanks in advance

when it started, rent paid date irrelevant.