In transit on USPS is not the same as out for delivery.
When USPS tracking says something is in transit it means that the package is in transit on a semi or other mail truck to be delivered to either another sorting facility or to your local post office.
When the package is out for delivery the USPS tracking will update to say out for delivery or arrived at your local post office.
Once you see out for delivery then the package should arrive on that day.
When USPS says your package is moving within the USPS network and is on track to be delivered to it's final destination and it's current in transit to the next facility it means that your package has been sorted and has been processed through the USPS sorting facility.
The package has then also been loaded onto a semi truck or other truck to be delivered to another sorting facility or your local post office.
The package is traveling down the road and is on it's way to you.
Yes the USPS mail does travel on weekends.
USPS operates the sorting facilities and over the road USPS trucks everyday of the week and the sorting facilities are operating and open 24 hours per day and 7 days per week including most holidays.
The USPS keeps mail traveling so that the mail can arrive on time.
Saturday is considered a business day for USPS.
Mail is delivered on Saturday and post offices are also open on Saturday.
Some Post Offices are open on Saturday for half the day while some larger post offices are open normal business hours as well.
Sunday is not considered a business day for USPS since the post offices are closed although some mail such as on rural routes or larger cities may get delivered on Sunday.
The USPS sorting facilities are open on Saturday and Sunday and most holidays as well and mail is being sorted 24 hours per day and 7 days per week.
Mail also travels on the weekends and even Saturday and holidays to get to you.