Does the drop-off day count as day 1?
No. The day the carrier accepts your package is day 0, and day 1 is the next day the carrier delivers. If you drop it off after the location's last pickup or acceptance time, the carrier accepts it the next day it's open, so your clock starts a day later.
Example: you drop off a USPS Ground Advantage package (2–5 days) on Friday, November 6, before the last pickup. Friday is day 0, Saturday the 7th is day 1, Monday the 9th is day 2, and Veterans Day on Wednesday the 11th is skipped, so day 5 is Friday, November 13. It should arrive between Monday the 9th and Friday the 13th.
Weekends
USPS counts Saturday as a delivery day and skips Sunday. UPS and FedEx count weekdays only. Both UPS and FedEx deliver to many homes on Saturdays, so a package can show up earlier than the count says, but never later because of it.
Holidays
USPS doesn't deliver on the 11 federal holidays, except Priority Mail Express. UPS and FedEx close on fewer of them: New Year's Day, Memorial Day, Independence Day, Labor Day, Thanksgiving and Christmas. That's why a package can move on Veterans Day or Columbus Day with UPS or FedEx but not with the Post Office.
Handling time isn't transit time
When an online seller says "ships in 2–3 days", that's handling time: how long before they hand it to the carrier. The carrier's clock only starts once it has the package, so add the handling time first, then the delivery days.
Cutoff times
Every Post Office, UPS location and FedEx drop-off has a last pickup time, often mid-afternoon, and blue collection boxes list theirs on the front. Miss it and your package is accepted the next business day. Tracking usually shows the first scan on the day it was accepted.
Questions people ask
Does the day you mail something count?
No. The day the carrier accepts it is day 0, and the count starts with the next delivery day.
Does USPS count Saturday as a business day?
Yes. USPS delivers on Saturdays and counts them, and it skips Sundays and federal holidays. UPS and FedEx count Monday through Friday.
What if I drop it off on a Sunday?
It's accepted the next day the carrier operates, usually Monday, which becomes day 0. Day 1 is the following delivery day.