Grinding is the process by which players gain experience by killing mobs. This is the main way character gain levels because quests do not offer enough experience to allow the character to level. One of the main complains the players have about 9Dragons is that grinding takes more time than all other aspects of the game.
There are events on Friday, Saturday, and Sunday where the experience rate is doubled, as well as the Lucky Cricket event on Wednesdays, which multiplies the experience rate by 2, 3, 4, or 5. This is the most efficient time to grind because you can reduce the amount of time spent grinding by up to a factor of 5.