It's only available on Fridays. When you first enter Union Cave, go to the last water area (on the bottom). The rest is pretty simple, just surf and walk. Until you get to a large area, with water and a bit of land in between.
A Lapras should be swimming in the last water area there. Just surf up to it and try interacting with it, and you will go into a battle with it, it's Lv 20.