Do weekend days count in 21 day waiting period for tax refund

Yes they do but don't bank on getting the refund in 21 days or less ... that is a goal not a guarantee.  

Directly from the IRS :   

https://www.irs.gov/refunds/tax-season-refund-frequently-asked-questions

How quickly will I get my refund?

We issue most refunds in less than 21 calendar days. However, it’s possible your tax return may require additional review and take longer. Where’s My Refund? has the most up to date information available about your refund.

It is taking the IRS more than 21 days to issue refunds for some 2020 tax returns that require review including incorrect Recovery Rebate Credit amounts, or that used 2019 income to figure the Earned Income Tax Credit (EITC) and Additional Child Tax Credit (ACTC).

I’m counting on my refund for something important. Can I expect to receive it in 21 days?

Many different factors can affect the timing of your refund after we receive your return. Even though we issue most refunds in less than 21 days, it’s possible your refund may take longer. Also, remember to take into consideration the time it takes for your financial institution to post the refund to your account or for you to receive it by mail.

It's been longer than 21 days since the IRS received my return and I haven’t gotten my refund. Why?

Some tax returns take longer to process than others for many reasons, including when a return:

• Includes errors, such as incorrect Recovery Rebate Credit
• Is incomplete
• Needs further review in general
• Is affected by identity theft or fraud
• Includes a claim filed for an Earned Income Tax Credit or an Additional Child Tax Credit. See Q&A below.
• Includes a Form 8379, Injured Spouse Allocation PDF, which could take up to 14 weeks to process

For the latest information on IRS refund processing during the COVID-19 pandemic, see the IRS Operations Status page.

We will contact you by mail when (or if) we need more information to process your return. If we’re still processing your return or correcting an error, neither Where’s My Refund? or our phone representatives will be able to provide you with your specific refund date. Please check Where’s My Refund? for updated information on your refund.

The IRS is really, really backlogged.  Some folks have been waiting since January.