Why do some Weekly Shonen Jump issues have two numbers?

A combined issue, printed with two numbers such as No.3-4 or No.22-23, is a single physical magazine that covers two weeks of the numbering sequence. Shueisha publishes them around the major Japanese holiday breaks, principally New Year, Golden Week in late April and early May, and Obon in August, when no issue appears the following week. The numbering never skips: the next magazine after No.3-4 is No.5, and the year still ends in the low fifties.

Collectors feel the second trap more than the first: the cover date is not the sale date. The date printed on a Jump cover is a cataloguing date, in the Dragon Ball years typically the Monday of the cover week. The issue actually went on sale about two weeks earlier, officially on Tuesdays until the switch to Monday release in May 2003, and the only place the true sale date was printed was in the preview in the previous issue. That is why the biggest issue of all, No.3-4 for 1995, carries a 1995 number but was released on December 20, 1994, and why the One Piece debut issue, 1997 No.34, is cover dated August 4, 1997 but went on sale July 22.

Read a Jump cover like a collector, then: the year and number identify the issue, the combined number tells you a holiday followed it, and the printed date tells you the catalogue week, not the day it reached a shop.