If I have a  massacre wurm in play ( http://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=214044 ) and then I wipe the board with a day of judgment ( http://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=208297 ) and my opponent has, lets say three creatures on his board, does he lose 6 life because of the wurm, or does the wrath effect kill him before i he gets the chance to activate?

Good Question. How it normally works is at the moment Day of Judgement resolves, all creatures are destroyed. As all creatures die, they hit the graveyard at the same time and massacre wurm's effect is triggered. That effect goes on the stack. Just because you destroy the source of an effect, doesn't stop the effect that's already on the stack. Therefore, after all creatures hit the graveyard, the ability resolves and checks the field for how many opponents creatures have hit the graveyard. So yes, you're opponent would lose 6 life. Also, a good note to point out is tokens hit the graveyard. Even though they don't remain in the graveyard, they do get "put into the graveyard". Once in the graveyard, that's when they dissolve and disappear. So if a token dies, massacre wurm would cause your aopponent to lose 2 life. Down below is the technical ruling found in the comprehensive rules. I hope this answered your question.

603.6d Normally, objects that exist immediately after an event are checked to see if the event matched any trigger conditions. Continuous effects that exist at that time are used to determine what the trigger conditions are and what the objects involved in the event look like. However, some triggered abilities must be treated specially because the object with the ability may no longer be on the battlefield, may have moved to a hand or library, or may no longer be controlled by the appropriate player. The game has to "look back in time" to determine if these abilities trigger. Leaves-the-battlefield abilities, abilities that trigger when a permanent phases out, abilities that trigger when an object that all players can see is put into a hand or library, abilities that trigger specifically when an object becomes unattached, abilities that trigger when a player loses control of an object, and abilities that trigger when a player planeswalks away from a plane will trigger based on their existence, and the appearance of objects, prior to the event rather than afterward.

Example: Two creatures are on the battlefield along with an artifact that has the ability "Whenever a creature is put into a graveyard from the battlefield, you gain 1 life." Someone plays a spell that destroys all artifacts, creatures, and enchantments. The artifact's ability triggers twice, even though the artifact goes to its owner's graveyard at the same time as the creatures.

Thank you very much rob, very helpful