From what I can tell, low tide is on Wednesdays, high tide is every day but Wednesday. Wesley of Wednesday stands in an area that's only accessible when the tide is low, so I'm pretty sure it's low tide all day Wednesday. There might be other times, but I'm not really sure.
And about when to do EV training, I usually do it in the mid-20s. Usually it's only newly-hatched Pokes that I EV train, so I take the parents out of the Day Care and stick the newborn in (being extra careful that I take it to the Move Deleter in Blackthorn and check its Learnset on Serebii to make sure I won't overwrite any Egg Moves I might have bred on it). Then I just pedal up and down Goldenrod as usual, hatching the other five eggs I most likely had in my party when it hatched. By the time I'm done it's usually in the 20s, so I Pokerus it up, equip the necessary Power Item and get to EV training.
I like to do it above Level 20 to make sure that whatever Pokes I'm fighting for EVs are guaranteed to be OHKOed by whatever move I'm using so I can keep track of them by looking at the PP I've used. An extra bonus is that when you can get all your EV training done without levelling up, then level up when you're done, you get to see your stats going up by 21 points or something :lol