They will tell you the clues now, I'm not sure they do on the actual Toy Day. It's random and they will always give you two clues for the item they want, for instance one might say green and toy or blue and carpet.
The villagers will describe the item they want to you in the coming days before Toy Day. They will describe the item they want using a colour and the type of item. There is no way to predict what items they will want, so it is random in that sense, however the items they want for Toy Day is pre-determined. There is no way to change what they will want for Toy Day.
I think once December starts the game gives each villager an item they want, throughout the month they will give you hints if you talk to them. 1 hint is normally color, and the other is the type of item (clothes, a toy, something electronic, etc.). I'd use the Game Notes app on your 3DS to record this. So yeah, 2 hints per villager, once December starts it's predetermined, you can only change it if you time travel out of December I reckon, but I'm not sure.