Actually, I've experimented a bit with the sequence on the Pokemon News on my Emerald. I've gotten a surprising pattern, too.
So, here it is.
1. Blend Master Arrival Broadcast - 10 days after beating the E4. (Game Time)
2. Mauville Game Corner "Service Day" - 20 days after the Blend Master. (Game Time)
3. Energy Guru Half-price Sale - 20 days after MGC Service Day. (Game Time)
4. Lilycove Clear-out Sale - 40+ days after E4. Very Rare event. (Game Time)
Of course, this experiment only involved my ONE Emerald game, so it isn't certain if this pattern is also reliable for the other R/S/E Games. But it won't hurt to try it out, though. If it doesn't work, then the event may randomly arrive sooner or later.