If I cancel, will the servers run til the end of the month or will they shutdown immediatly?

When you cancel the servers, the time for the service stops immidiately and shuts down. At the end of your billing cycle, your statement will reflect a credit for the time not used on the server. You may then get a refund for that time applied back to the payment method(s) used to make those payments.

In case of donations, payment is applied to the account in this order:

Personal payment, then donations in the order they were provided.

So if you have donations, the donors, starting with the most recent will recieve what they provided back to their payment method used, and then the next recent, and so forth until the money has been provided to who made the payments.

This is due to two reasons:
1) Refunds are processed as true refunds. 
2) In some countries, legally, those funds can only be used for the costs of which they were gathered. So to avoid potential legal pitfalls, all the funds are returned to their respective parties.