I'm currently trying to obtain all the Mega Stones, but I accidentally forgot to pick up the Ralts carrying a Gardenvoirite from the Champion at the coffee shop in Lumiose City. I'm pretty sure I went to the right cafe? Is there ans exact time/day?
Yes, you can still get it.
You should be going to the Cafe Soleil (forgive me if I misspelled that). Ride a Lumi Cab if you can't find it. There is no required time/date, and she will remain in the cafe until you trade with her. She won't disappear.