Which activity or event is the origin of the expression ‘The sun is over the yardarm’?

The expression ‘The sun is over the yardarm’ is widely believed to have been a nautical term, originating in the late nineteenth century on board British naval ships in the North Atlantic Ocean.

Due to their position, certain ships that sailed in the North Atlantic Ocean would often experience the sun rising above the upper mast spars (yards) of their square sails around 11 am. This coincided with the forenoon ‘stand easy’ announcement, which was when the officers would go below and partake of their first rum tot of the day. The ‘stand easy’ also allowed the crew to take a break from their duties, although coffee was the strongest drink they were allowed, in contrast to the more potent libations enjoyed by the officers.

The phrase has since been adopted across the English speaking world as a social metaphor describing a suitable time to have the first alcoholic beverage of the day.