Currently a minimum mixin of 2 is mandatory. It is my understanding that the mandatory minimum mixin will be increased to 4 at some point. When will that happen?
Block 1406997 doesn't come for 341.5 days (as of this writing, Oct 19th, 2016, at block 1161101). September 25th of 2017 seems a bit...much. Please correct my errors.
(I would have posted as a comment to the answer above, but my rating on stackoverflow is too low. You could help by up-voting me, just for trying to be constructive.)
This will happen on version 5, at block height 1406997 on mainnet (it's already happened on testnet).