Reading in between the lines of Gatsby's "glancing" which caused his house to be so lit up, we later find that he could have been preparing, just in case Daisy comes by after the tea. From what we learn of this guy, we realize he knows he needs to be a little vain to gain Daisy's approval.
On another note, that reference or allusion to the World's Fair is a little ironic. Gatsby's house regularly contained visitors and catering for such an event like the World's Fair. This was another feature that drew Daisy's attention.
You can find the answer for this right at the very beginning of Chapter 5. The answer is that Gatsby's house is all lit up very brightly and so it looks, to Nick, as if it is the World's Fair.
When Nick first gets to West Egg that evening, things are so bright that he thinks his own house must be on fire. But actually what is going on is that every room in Gatsby's house has its lights on. When Nick asks Gatsby about it, Gatsby tells him that he was just "glancing" into the various rooms.