RN lingo help?

[Mush Martin]

What is the correct Royal Navy or Commonwealth slang
for the watch that goes off duty at midnight?

What is the correct Royal Navy or Commonwealth slang
for the Watch that goes on duty at midnight?

Please and Thanks
M

[TarJak]

First Watch ends at midnight and the Middle Watch begins at midnight. Most Commonwealth Navies follow this timetable also as they derived most of their customs and attributes from the RN:

Watches in the Royal Navy are named as follows:
First Watch8pm to midnight2000 to 0000
Middle WatchMidnight to 4am0000 to 0400
Morning Watch4am to 8am0400 to 0800
Forenoon Watch8am to noon0800 to 1200
Afternoon WatchNoon to 4pm1200 to 1600
First Dog Watch4pm to 6pm1600 to 1800
Last Dog Watch6pm to 8pm1800 to 2000

The "Relieve Decks" is worked by the Officer(s) of the Watch only from 0730 to 0800. Each watch is of four hours, except the "dog watches" which are two hours. A bell is rung every half hour, and the total number of bells in a watch (except the "dog watch") is therefore eight. Eight bells announces the end of a watch. One bell announces that half an hour has passed, and so on to its end.

Members of a watch can then tell, from the number of bells sounded, just how much of their watch has passed.

Dog Watches The name probably comes for DODGE WATCH: by making in this way a total of seven watches to the day, men would be enabled not to keep the same watch each day. The suggestion that the name DOG comes from a dog watch being a watch cur-tailed is too frivolous to be authentic. A dog watch being two hours long while all other watches are of four hours' duration gives rise to the common Naval expression of derision to a junior: "You've only been in the Navy a dog watch".

The custom of striking 1-2-3-8 bells in the last dog watch, instead of 5-6-7-8, is said to have originated in 1797; the mutineers at the Nore timed their mutiny to start at "five bells in the dog watches" on 13th May, 1797, but the officers got to hear of this and directed that five bells should not be struck then. Since then, one bell has been struck at 6.30pm. Some foreigners still carry out the old routine, but most have come into line with us.

In the Royal Navy, the two Dog Watches are the "First" and the "Last" not the "First" and the "Second". In everyone's mess but nobody's watch An old Naval expression used to describe a man who talks a lot but avoids actual work as much as he can - a good hand in the canteen but never available when there is work to be done.
Copied from: http://www.royal-navy.mod.uk/server/show/nav.3827
Which has most of the RN slang known to man.