Counting days for tax levy and/or garnishments

I know the letter to the customer is within two business days. For a 21-day tax levy do you count 21 calendar days or 21 business days? Also, do you count the date of receipt as day one or the next day as day one? How do you count days for a garnishment? I feel like I already know this, but I've read so much that I can't recall it or find it on paper or computer. Thanks in advance!!

The IRS Notice of Levy Form 668-A specifically says "21 calendar days." The date the levy is received is "Day 0."

Thanks!