How long does it take for the FCC to process vanity callsign applications?
If you submit your application online on a federal workday (Monday to Friday, except holidays), even after hours, it is considered received that day. If you submit your application on a weekend or a holiday, it will be considered received on the next federal workday.
There is a waiting period of 17 days, during which applications aren't processed. This allows time for mailed-in applications to be processed (yes, they're still allowed).
If the 18th day (inclusive) is a federal workday, your application will be queued for processing that night. Otherwise, it will be queued for processing on the night of the next federal workday.
The results of your application will be posted early in the morning of the following day (regardless of whether it is a federal workday).
Because the receipt date and the processing date must both be federal workdays, and be separated by 17 days, it works out that processing will only happen on Monday (if the receipt date was Wednesday through Friday), Thursday (if the receipt date was Monday), or Friday (if the receipt date was Tuesday), and the results will show on Tuesday, Friday, or Saturday mornings.
Normally, this works out to an 18-20 day wait, but holidays may increase it to 21. I don't think any two federal holidays are perfectly spaced to create the perfect storm of a 24-day wait.
Most information in this answer sourced from AE7Q.