AVIVA only accept contributions on a calendar monthly basis. You should ensure payroll is set up to send pension contributions on this basis. Some payroll software can "roll-up" weekly, fortnightly or lunar contributions in to monthly submissions. Check with your software provider if you're not sure.
In a lot of cases you may find that the PensionSync Automatic Date Matching will help avoid errors where slight variations on these settings are used (e.g. payroll uses Tax Periods).