P.4.1- This phase has three steps: Untap, Upkeep, and Draw. In that order. [CompRules 1999/04/23]
P.4.2- The "mana burn" (see Rule P.3.5 and Rule G.22) that happens at the end of this phase takes place at the end of the final step of this phase. Normally, this is the Draw step, but if the Draw step is skipped, then mana burn happens at the end of the Upkeep step. [D'Angelo 1999/05/01]
P.4.3- Any abilities triggering "at beginning of turn" trigger at the beginning of the first step of the turn. Normally this is the Untap step, but if that step is skipped, it is the Upkeep step. [D'Angelo 1999/05/01]
P.4.4- Any static abilities which change their effect when the current player changes (such as Vibrating Sphere), take effect at the start of the first step of the turn. Normally, this is the Untap step, but if that step is skipped, it is the Upkeep step. This takes place before checking for any triggered abilities during that turn. [D'Angelo 1999/05/01]
P.4.Ruling.1- If an effect to skip a turn is optional, make the choice right before the turn starts. [D'Angelo 1996/11/06] If more than one effect can cause you to skip a turn, choose which one is making you skip just before you would start the turn. [Barclay 1998/07/29]