About Holy Water
On Theophany, that is, the Day of the Lord's Baptism, every year a great miracle is performed. The Holy Spirit, coming down upon the water, changes its natural properties. It becomes incorrupt, that is it does not spoil, remains transparent and fresh for many years, receives the grace to heal illnesses, to drive away demons and every evil power, to preserve people and their dwellings from every danger, to sanctify various objects whether for church or home use. Therefore Orthodox Christians with reverence drink Holy Water - a great Agiasma (holy thing), as the Greeks call it.
One should always have at home enough Theophany water so that it will last the whole year, and make use of it at every need; in cases of illness, leaving on a journey, whenever one is upset, students when going to examinations. They do well who daily, before eating any kind of food, drink a little Holy Water. It strengthens the powers of our soul - if it is done, of course, with prayer and reverence, and one does not merely expect from it a mechanical result.
Every priest should take care to bless a sufficient quantity of water for his church, so that it will be on hand for the course of the whole year for every need and to be given out to those who ask for it; and parishioners should provide for themselves at Theophany with Holy Water for the whole year and even so that it can be kept for future years.
St. John of Shanghai
Source: Pravoslavie.ru