If the fixed-term is 90 days or less, it is a short fixed-term tenancy. It does not become periodic when the term ends.
A fixed-term tenancy agreement lasts for a set amount of time – eg, one year. Once the term ends, the tenancy will become periodic. If you don’t want this to happen, you’ll need to give notice in writing at least 21 days before the term ends.
A periodic tenancy agreement has no end date. It continues until either the tenant or the landlord gives written notice to end it.