What are the usual Quarter Days?

These are often the dates specified in leases when the rent and any service charges are payable and they are:

Lady Day: 25th March.
Midsummer Day: 24th June.
Michaelmas Day: 29th September.
Christmas Day: 25th December.

Rent and service charges are often paid on the usual Quarter Days in advance, that is for example the rent due on 24th June is for the period from 24th June to 28th September inclusive. In some old leases payment can be in arrears, that is for example the rent due on 24th June is for the period from 26th March to 24th June inclusive.

What happens if I don’t pay the rent?

This would be a breach of your obligations under the terms of your lease, and might enable the landlord to end the tenancy. Landlords have a common law right, as soon as rent is overdue, to enter tenanted property and to seize and hold goods found there until the rent is paid. Persistent delay in paying rent would give your landlord a ground of opposition if you were to apply for a new tenancy.