User blog comment:FairyRaven/Fairy Tail Chapter 328: Zodiac, Review and Commentary/@comment-180.190.247.74-20130426000018/@comment-4829046-20130426062511

A double issue doesn't mean that the contents are doubled. During four particular weeks of the year, Japan celebrates major holidays and festivities, for example, the Golden Week in May. These are the only times where the mangaka get a break, so the magazine also goes on break for a week. When this happens, the preceeding issue is a "double issue". So, a double issue means the magazine is meant for a 2-week period, which implies that there will be no WSJ issue in the following week.

Its cause of Golden Week. It happens every year. There is also a silver week. You don't read that many manga do you?