Half-mast or half-staff describes a flag flying approximately halfway up a flagpole or ship's mast. This is done in many countries as a symbol of respect, mourning, or distress.
The tradition of flying the flag at half-mast began centuries ago, to allow "the invisible flag of death" to fly at the top of the mast-which signified death's presence, power, and prominence. In some countries, for example the UK, and especially in military contexts, a "half-mast" flag is still flown exactly one flag's width down from its normal position, and no lower, to allow for this flag of death.
When hoisting a flag that is to be displayed at half-mast, it should be hoisted to the finial for an instant, then lowered to half-mast. Likewise when it is lowered at the end of the day, it is to be hoisted to the finial for an instant, and then lowered.
The term "half-staff" is commonly used to refer to half-mast, although military tradition indicates that "half-mast" is generally reserved to usage aboard a ship, where flags are typically flown from masts. Not all English-speaking nations observe this distinction. (wikipedia)
The Government is responsible for the decision on the occasions on which the flag should be flown at half-staff.