As Blondie posted, it seems to be a substitution for the 7 appointed times in the Scriptures. Granted, during the early years there may have been a connection (as in they were 5 days or longer, possibly a few at the same time as Feast of Tabernacles).
More so, today, to hype of releasing new literature (or the recent "Silver Sword"/"sister proof" RNWT). Also hype time toward baptisms, meeting others, and getting all "dignified" dressed with dry cleaned clothes.
Still it comes down to $$, as in a goal of contributions > true cost of the assembly. And good PR for bringing business to the city.