Operating schedules of U.S. broadcasting stations Unlimited Time ("U"): Operation is permitted 24 hours a day, 7 hours a week. Station is required to operate at least 8 hours between 6am and 6pm, and at least 4 hours between 6pm and midnight. Class D stations need only meet this regulation for the first period, 8 hours between 6am and 6pm. A majority of AM stations are licensed for unlimited time (however, many are Class D) as are virtually all FM stations. (there are a tiny handful of shared-time stations on FM)

Share Time ("ST"): Two stations share time on the same frequency. The FCC may specify the division of time in the stations' licenses, or the stations may reach a private agreement which is then filed with the FCC and considered part of their licenses.
It is possible for more than two stations to share a channel. No such situation currently exists, though there was a three-way sharing arrangement in Chicago well into the 1990s. It is also possible for stations to share time at night and operate simultaneously during the day. (when they are far enough apart that simultaneous daytime operation won't cause interference) Again no such situation currently exists. At one time, KFAB and WBBM operated simultaneously on 780 - with separate programming during the day, and synchronized simulcasting at night.
Share-time stations are required to operate at least 2/3 of their authorized hours between 6am and 6pm, and at least 2/3 of their authorized hours between 6pm and midnight.

Daytime ("D"): Operation at full power is permitted between average monthly sunrise and average monthly sunset. The FCC attaches a document (obtained from the U.S. Naval Observatory's American Nautical Almanac) to the license specifying these sunrise and sunset times. The station must operate at least 2/3 of its authorized hours between 6am and 6pm, and at least 2/3 of its authorized hours between 6pm and midnight. (Much of the year, daytimers have no authorized hours after 6pm, so the latter period doesn't apply.)
In most cases, daytime stations may begin operation under "Presunrise Service Authorization" (PSRA) at 6am. Stations within the 0.5mV/m contour of a Class A station to the east must wait until sunrise at the Class A station. (this means that if sunrise at the Class A station is after 6am, the daytimer has no PSRA) This authorization is at no greater than 500 watts, and usually quite a bit less.
Daytime stations also usually have "Postsunset Service Authorization" (PSSA). This too is at no greater than 500 watts and usually less. PSSA operation begins at sunset and in most cases continues for two hours. Stations within the 0.5mV/m contour of a Class A station to the west must stop PSSA operation at sunset at the Class A station. (this means that if the Class A station is *east* of the daytimer, no PSSA operation is possible)

Limited Time ("L"): Limited time stations are allowed to operate during daytime, and until sunset at the Class A station on the frequency if the Class A station is to the west. A limited time station is also permitted to broadcast during any nighttime hours that the Class A station doesn't use. (probably irrelevant today with all Class A stations operating 24/7, but when most stations had overnight silent periods it was quite applicable) Limited time stations must operate at least 2/3 of their authorized hours between 6am and 6pm, and at least 2/3 of their authorized hours between 6pm and midnight.

Specified Hours ("SH"): The operating hours are specified in the station's license. Additional hours are permitted for special events. While the FCC regulations don't say so, this category seems to allow for stations in sparsely-populated places where it's economically impossible to meet the 12 hours a day minimum schedule. There are very few specified hours stations in existence today, in fact I'm unable to find *any* in the NRC Log.
The regulations are somewhat unclear as to whether the hours specified in the license are a minimum, or if the station is only required to operate 2/3 of the hours specified. I suspect the figures in the license are a minimum.

No radio station is required to operate on Sunday. I know of one station (WDMP AM/FM) which at one time took advantage of this regulation, and was always silent on Sundays.

No minimum schedule applies to non-commercial AM or TV stations. In theory, a limited operating schedule can be held against the station in renewal - however as a station can no longer lose its license except for grevious violations of the regulations, this would no longer seem to apply.

Non-commercial *FM* stations must operate at least 36 hours a week, and at least 5 hours a day at least 6 days a week. Stations licensed to educational institutions, however, need not operate on weekends or when school is not in session.

Non-commercial FM stations that don't operate at least 12 hours every day are subject to time sharing, upon application by someone else for the same frequency.

All TV stations are licensed for unlimited time, though there have been share time TV operations in the past. However, the minimum schedule for TV is different:

In the first 3 years of operation, TV stations must operate at least 2 hours a day and 5 days a week. They must also operate at least 12 hours a week during their first 1-1/2 years on the air; this figure increases by 4 hours every six months after that.

After 3 years of operation, TV stations must broadcast at least 2 hours every day (no Sunday exception) and at least 28 hours a week. Test patterns don't count.

Stations are required to notify the FCC within 10 days if they will not be able to meet the minimum operating schedule. (for example, if the tower collapses and they won't be able to operate at all) Such notification allows a 30-day silent period, which may be extended by further notification.

An Act of Congress (not a FCC regulation) requires the FCC to revoke the license of any station which remains off the air for over a year, even if the Commission was notified of the outage. However, there seems to be a loophole in this regulation: it doesn't seem to specify any minimum operating period between silent stretches. Some stations have been known to operate for just a day or two every few months simply to keep this law from kicking in.

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