Complex sentence: Guru-da-Gaddi is an important Sikh religious event held on the 3rd of November, commemorating when the tenth (and last) Guru, said'the next Guru would be the Holy Sikh Book' Guru Granth Sahib.
Split into simpler sentences: Guru-da-Gaddi is an important Sikh religious event held every November 3. The event honors when the tenth and last (Sikh Gurus) when they said'the next Guru would be the Holy Sikh Book' Guru Granth Sahib.