You send invoices to your client
Important is the date of the invoice. The moment you create the invoice you owe the VAT which you have mentioned on the invoice to the tax authorities. So if you have created an invoice on December 28, the VAT on this invoice will have to be declared in the VAT return of the 4th quarter. Even if the invoice is only paid on, for example, February 19 of the following year. You will still have to pay the VAT to the tax authorities although the client may not have paid you yet at that moment.
To prevent that companies delay the payment of VAT by issuing invoices with a later date, the VAT law states that an invoice must be sent to the client before the 15th day of the month following the month in which the product was delivered or the service provided. If you provide ongoing services you must also invoice on a regular basis.
Your clients pay you cash
If you mainly deliver to consumers you will most of the times receive the payment immediately, either cash or by bank transfer. The moment you receive the payment the VAT becomes payable to the tax authorities and will have to be declared in the VAT return of that quarter.