The IRS doesn't say whether the 21 days is business days or all days. They just say that most refunds are issued within 21 days or less. They also say that some refunds take longer. The only way to know for sure when you will get your refund is to track it on the IRS web site: https://www.irs.gov/refunds
It may take a few additional days for your bank to post the refund to your account. If you are having your fees taken out of your federal refund, that will add a few days to the time it takes for the rest of the refund to get to your bank.
