So just a quick question about sneak attack. I’m assuming from the errata and the newest wording of the card that I can use this ability even when it is not my turn, to cast a creature for blocking purposes. Is this correct?
In general, you can activate an activated ability (such as Sneak Attack's ability) any time you have priority, even if it isn't your turn (C.R. 117.1b, 117.1, 117.3), unless the ability states otherwise (for example, if it says "Activate only during your turn" or "Activate only as a sorcery" [C.R. 307.5]; examples include Hoofprints of the Stag and Whip of Erebos) (C.R. 602.1b). This is true regardless of whether the ability appears on an enchantment or not. See also this thread, this thread, and this thread. In the case of Sneak Attack, that card didn't recently experience any changes to its text that would affect this answer.
Note, however, that Sneak Attack's ability doesn't have you cast a creature card from your hand; rather, you may choose to put a creature card from your hand onto the battlefield when that ability resolves, which is not the same thing (review C.R. 701.4a).
EDIT: Correctness edit after comment 4 was posted.
EDIT (Apr. 20, 2021): Edited to conform to rule update with Strixhaven.
So just a quick question about sneak attack. I’m assuming from the errata and the newest wording...
This card has worked the same since it was created, 22 years ago. It has always been able to spring out a surprise blocker.
The new wording only allowed to streamline some weird corner case scenarios, should a player have the strange idea to activate it, and then refuse to play out a creature.
In general, you can activate an activated ability (such as Sneak Attack's ability) any time you have priority, even if it isn't your turn (C.R. 117.1b, 117.1, 117.3), unless the ability states otherwise (for example, if it says "Activate only during your turn" or "Activate only as a sorcery" [C.R. 307.5]; examples include Hoofprints of the Stag and Whip of Erebos) (C.R. 602.1b). This is true regardless of whether the ability appears on an enchantment or not. See also this thread, this thread, and this thread. In the case of Sneak Attack, that card didn't recently experience any changes to its text that would affect this answer.
Note, however, that Sneak Attack's ability doesn't have you cast a creature card from your hand; rather, you may choose to put a creature card from your hand onto the battlefield when that ability resolves, which is not the same thing (review C.R. 701.4a).
EDIT: Correctness edit after comment 4 was posted.
EDIT (Apr. 20, 2021): Edited to conform to rule update with Strixhaven.
The new wording only allowed to streamline some weird corner case scenarios, should a player have the strange idea to activate it, and then refuse to play out a creature.
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules