It's because the lunar cycle
is 28 days. Which splits nicely into 4 sevens.

Having a day off in every seven must have seemed like a good idea to the ancients (seeing as, allegedly, God had the idea first). But we modern types have extended it to 2 days in 7.

It's not universal though. Lots of folk work odd patterns. I've a friend on the rigs who works 12 hour shifts for 2 weeks, then has 3 weeks on shore.
( K2k6 has a proper job these days, Wed 7 Jul 2010, 11:52, 3 replies, latest was 15 years ago)

"On the Rigs", is that like "On the buses"
but with less 1970's saucy antics and more oil soaked pelicans?

"I'll get you deepwater horizon!"
( Colonel Dracula Two manky hookers and a racist dwarf, Wed 7 Jul 2010, 11:59, Reply)

It's more 'On the Choppers'
as they have to fly out there in helicopters. He's told me a few hair raising stories about helicopter trips.
( K2k6 has a proper job these days, Wed 7 Jul 2010, 12:00, Reply)

I bet they're "on the choppers", all those men in a remote confined place for long periods
They work hard and they play hard...EVERYBODY DANCE NOW!
( Colonel Dracula Two manky hookers and a racist dwarf, Wed 7 Jul 2010, 12:07, Reply)

Like it, Colin, like it A LOT.

( Monty Boyce, My cheese game is strong, Wed 7 Jul 2010, 13:36, Reply)

My company almost did a crazy shift pattern which would've given me 7 consecutive days off every month,
I could've handled that.
( Undulating Tentacles of Love getting fun down to acceptable levels, Wed 7 Jul 2010, 12:01, Reply)

My dad used to work 6 months on 3 months off
when he was on a ship and the shifts were 12 hours each.
( Kitty v1.0 desperately naive, Wed 7 Jul 2010, 12:13, Reply)

my neighbour used to do that
on the rigs - 6 months on, 3 off when based on North Sea rigs. then he changed company and was 6 on 6 off in places like texas and nigeria
(bamboozled Can hear you getting fatter, Wed 7 Jul 2010, 12:24, Reply)

I don't think I'd mind that too much
especially since my dad loved his job so it wasn't like an epic Monday morning having to go back.
( Kitty v1.0 desperately naive, Wed 7 Jul 2010, 12:29, Reply)

My father got 6 months' holiday a year.

( Monty Boyce, My cheese game is strong, Wed 7 Jul 2010, 13:37, Reply)