User blog comment:Miskos3/Fairy Tail Chapter 334: Sin and Sacrifice, Review/@comment-203.215.121.20-20130608014243/@comment-5840892-20130608051230

Seven is the most powerful number in Magic. There's a lot myth behind the number 7. One is:

The 7th Hour, of the 7th Day, of the 7th Month, is the time when Magic is strongest.

Mashima is not the first to make 7 so important in a series about Magic, J.K. Rowling did so to. She even directly reffereneced it in one of the books.