Proper Japanese Title: 週刊ヤングサンデー
Weekly Young Sunday was Shogakukan's equivalent to the major seinen magazines from Kodansha and Shueisha (Weekly Young Magazine and Weekly Young Jump respectively). Young Sunday came along nearly a decade after the other two and like those magazines, Young Sunday was not actually weekly from the start and only became weekly at the end of 1995.
The biggest difference from those two magazines however is that Young Magazine and Young Jump launched as independent magazines but Young Sunday was actually a rebranding of a twice-ended shonen magazine (originally known as Manga-kun and then later Shonen Big Comic).
Young Sunday was ended in 2008 but many of its creators remained associated with other Shogakukan magazines and its role was essentially assumed by Big Comic Spirits (Shogakukan's first weekly seinen magazine).