At the beginning of each player's precombat main phase, if Blinkmoth Urn is untapped, that player addsto his or her mana pool for each artifact he or she controls.
10/4/2004 The precombat main phase is the first main phase of the turn. All others are postcombat main phases, even if they technically occur before combat.
12/1/2004 The “precombat main phase” is the first main phase of the turn. All other main phases are “postcombat main phases.”