Well, if the late Eric Newby was a member of this forum he'd know the answer straight away! So does (or should ) anyone who has read his book The Last Grain Race (or any similar book).
The keyword here is Sunday!
This day usually was a sailors' day off! The rest of the week the watches were occupied with all kinds of chores. Not a minute was wasted. All this accompanied by chronic hunger and fatigue. A sailors' life was hard.
But then there was Sunday. Apart from sailing the ship (obviously) no other work had to be carried out. Finally they had time for themselves. Cleaning, laundries, mending clothes, model making or just relaxing were favorite pastimes. Depending on the cook even the food might be a little more special. To put it simple: Sunday was sacred!
Imagine crossing the IDL from east to west. An extra day would then be added. If they crossed on a Sunday, the next day again would be...Sunday. Great!
Now imagine crossing the IDL from west to east. A day will be subtracted. Saturday is immediately followed by Monday. No Sunday at all....OMG. It all depended on the good will of the captain to provide any compensation.
I guess Subfixer kind of answered the question, he came up with Sunday. Let's move on, this was a bad question. Subfixer you're next!