Im in Emerald, and Im waiting for the rooftop sale in Lilycove's Department Store. How can I tell when it'll show?
"Randomly, after beating the Elite Four, Lilycove Department Store will advertise their rooftop sale by using the Pokémon News station. The times where a sale goes on is quite rare. However, the items sold in the sale cannot be received by any other method, other than from the trader."
So basically check out the Pokemon new station if you want to know when sales will start.