The sun is over the yardarm

 
A yardarm is long piece of timber tapering slightly toward the ends, hung by the centre to the top of a mast (upon which the square sails are traditionally hung).
The suggestion that one can have a drink as soon as the sun is over the yardarm is thought to have its origins in the custom aboard ship whereby once the sun had sunk enough over the horizon and no longer struck the yardarm officers could retire below for their first tot of spirits for the day. The expression is now used to mean around 5 p.m. or the end of the working day.