P.13.1- During this step the following actions are taken in order: a) If the current player's hand contains more cards than their maximum hand size (normally 7), they discard enough cards to reduce their hand size to that number. This action does not go on the stack. b) Simultaneously remove all damage from permanents, and all "until end of turn" and "this turn" effects end. This action does not go on the stack. c) If the conditions for any state-based effect (see Rule T.11) exist, or if any abilities have triggered, the active player receives priority to play spells or abilities after resolving those state-based effects and putting those triggered abilities on the stack. Once the stack is empty, instead of proceeding to the next turn, another Cleanup step begins. If no state-based effects need to be applied and no triggers need to be played, the step ends. [D'Angelo 1999/05/01]
P.13.Ruling.1- You may not just choose to discard because you want to. You only do so if you have more than 7 cards or because a spell or ability tells you to do so. [D'Angelo 1998/02/03]
Note- Remember to check for mana burn at the end of the phase. See Rule P.3.5.