When will our 1st payment be?

Had our mortgage offer from Santander finalised this weekend and will be moving into our property on April 5th. Mortgage payment is due to be on 1st of the month.

Does anyone know when our first payment will be? May 1st? If so will this be a double payment?

Just trying to plan finances in advance. Thanks

Will obviously ask and clarify with the bank but just wondering if anyone has an idea.