Complex sentence: This is backed up by the fact Arthur's Day has been held on a Thursday since its inception, rather than sticking to the original date, Thursdays being the traditional "student night" in Dublin and many towns in Ireland.
Split into simpler sentences: This is backed up by the fact that Arthur's Day has been held on a Thursday since its inception, rather than an accurate yearly basis on the original date. Instead, Thursday is chosen, Thursdays being the traditional "student night" in Dublin and many towns in Ireland.