Normal business hours are weekdays 0800-1300 and 1400-1700, Saturday 0800-1200. Government working hours are weekdays 0830-1230 and 1330-1630. Banking hours are weekdays 0900-1530. Post offices are open weekdays 0830-1600. Almost everything is closed on Sunday.
Due to its position just west of the international date line, Tonga is the first country in the world to usher in each new day. In fact, the date line seems to have so confused the local roosters that they crow constantly just to be safe.
Tonga shares its day with Fiji, New Zealand, and Australia, but is one day ahead of Samoa, Niue, Tahiti, and Hawaii. The time is the same as in Samoa, but one hour behind Hawaii and one ahead of Fiji and New Zealand.
Tonga's sacred Sunday adds to the fun of the date line confusion. Commercial flights (and most other transport) are banned in the kingdom that day. Don't get worried if you suddenly realize that it's Sunday on a flight to Tonga from Apia—you'll land on Monday. Just keep repeating: "If it's Sunday for the Samoans, it's Monday for the monarch."
Entertainment-wise, Friday is a much bigger party night than Saturday because most Tongans get paid on Friday, so there's money to spend and the fun doesn't have to stop at midnight. Only holy rolling is allowed on Sunday, but some bars, discos, and cinemas reopen a few minutes after midnight on Monday morning and try to make up for the time lost on the Sabbath. Expect a lot to happen late in Tonga.