Complex sentence: Guru-da-Gaddi is an important religious event for Sikh, commerating when the tenth Guru stated that the next Guru would be the Holy Sikh Book Guru Granth Sahib.
Split into simpler sentences: Guru-da-Gaddi is an important occasion of Sikh Religion. This is when the tenth Guru of the Sikh Religion had stated that the next Guru would be the Holy Sikh Book Guru Granth Sahib.