It is best to EV train before and then put the Pokemon in the Daycare after.
Basically there is a vague reason:
The Daycare will never know which stat you want to EV train in, so
they will EV train randomly at no pace. That is pretty much it. You
can have MAX of 510 EV's in one Pokemon and MAX 252 in one Stat. If
one stat is already maxed out, it will not need any more EV training.
EV Training Guide then How to EV train. and Youtube Video on EV training. also there is