The lucky chance doubles your luck chance rate. (When you merge any item, you automatically have a chance of having a lucky merge) you should be merging all items by 5. If you merge by 5 instead of 3, you will get automatically two items. Anytime though, you have a chance of getting lucky and you would get 3 items instead of 2 out of a 5 merge.
The event will double your luck, so you get an increased chance of getting extra lucky merges. I'm currently saving my keys for that event. Theoretically, it is hard to save all items for the events - but if you have a lot of space, you could save gazebo or greenhouse parts too. The luck chance is unknown to the players, so you don't know when a lucky merge would come.
*At this time, I'm not sure if it's worth it to merge greenhouse/gazebo parts for double xp, or if it is worth it to save for the lucky merge event. I am probably going to just merge my gazebo/greenhouse for the XP to get the XP and try to level up quicker. But if you want the gazebo/greenhouse parts rather than ranking up, then you can do opposite and save those for the lucky merge event.