"Curfew" explained

Anyone who has listened at any length to coverage of the developments in Egypt has heard numerous references to "curfew" being imposed (and ignored).  It's quite an interesting word; we begin with the etymology from the OED:

Etymology: < Anglo-Norman coeverfu, = Old French cuevre-fu, quevre-feu, covre-feu (13th cent.), < couvre, imperative of couvrir to cover + feu fire...

and the definitions:

1a. A regulation in force in mediæval Europe by which at a fixed hour in the evening, indicated by the ringing of a bell, fires were to be covered over or extinguished; also, the hour of evening when this signal was given, and the bell rung for the purpose.

1b. Hence, the practice of ringing a bell at a fixed hour in the evening, usually eight or nine o'clock, continued after the original purpose was obsolete, and often used as a signal in connection with various municipal or communal regulations; the practice of ringing the evening bell still survives in many towns. In extended use: a restriction imposed upon the movements of the inhabitants of an area for a specified period. The primary purpose of the curfew appears to have been the prevention of conflagrations arising from domestic fires left unextinguished at night. The earliest English quotations make no reference to the original sense of the word; the curfew being already in 13th c. merely a name for the ringing of the evening bell, and the time so marked. 

More details (and the image above) from Chambers' Book of Days, via Scribal Terror:

In those old times, people, in general, possessed nothing like clocks or watches; they learned, by the practice of observation, to judge roughly of the time of the day, but in cases where it was necessary to know the exact hour, they were entirely at a loss. Any implement for measuring time was rare, and belonged only to a public body, or institution, or to some very remarkable individual, and the only means of imparting to the public the know-ledge gained from it, was by ringing a bell, or blowing a horn, at certain hours of the day. This practice was first introduced in the monastic establishments..

It was attempted, in towns especially, to meet this evil [nocturnal lawlessness], by making it criminal to be found out of doors after a certain hour; and, as otherwise offenders might plead ignorance, it was ordered that the hour should be publicly sounded, generally by the town-bell, and when that was heard, all people were compelled to shut the doors of their houses, put out their fires, and retire to bed, those who were out of bed after the sounding of the bell being liable to severe punishment. It was an efficacious way of clearing the streets. The bell sounded for this purpose was, in France, called popularly the couvre-feu, or cover-fire...

The curfew is still rung in many towns and parishes in England, in some at eight o'clock, and in others at nine. At the end of the last century, as we learn from the Gentleman's Magazine for 1790, the curfew was announced at the latter hour at Ripon, in Yorkshire, by a man with a horn, which he blew, first at the market-cross, and then at the mayor's door...

It is quite a mistake to suppose that the curfew-bell was peculiar to this island—it was a natural expedient for serving a generally useful purpose, and was adopted in France, Italy, and Spain, and probably in all parts of continental Europe. More-over, a corresponding bell was rung in the morning, to inform people of the hour at which it was customary to rise...

And  finally, some tidbits from Elfinspell: the Curfew Bell, from Old Church Lore:

In England, the curfew law is said to have been made an established institution by King Alfred. When that monarch restored the University which had been founded at Oxford by St. Frideswide, he ordained, among other thoughtful regulations, that a bell should be rung every night at eight, when all the inhabitants of Oxford should cover up their fires and go to bed. The intention was not that the fires should be put out, but merely deadened. As Mr. Lomax observes, “The old fires were made in the centre of a large hearth, and the accumulated ashes were swept to the back and sides. At the curfew, the large logs were removed, and the cold ashes raked over the fire so as to cover it. A fire so covered will often keep smouldering for days, and can be re-lighted by adding fuel and admitting air, a most important consideration in the days of tinder-boxes. The same custom is still pursued in the backwoods of America, in the Australian bush, and in our own ‘black country,’ where the great coal fires are ‘raked’ in the old fashion nightly.”..

“The curfew tolls the knell of parting day...” [Thomas Gray's Elegy written in a Country Churchyard].