Correct me if I'm wrong but I think the history goes something like this.
James II, the legitimate monarch, had to flee to France after a group of English nobles in an act of treason and/or bravery invited a Dutch leader to invade England. James II attempts to put down the usurpers -- William of Orange and his protestant wife Mary (James' disloyal daughter) -- by gathering an army that will include a bunch loyal Irish, who happen to be catholics. But James is defeated and William of Orange is victorious and ever since then the protestants in Northern Ireland have been banging pots, marching about, playing the same three songs and lighting bonfires. Since 1690.
Many protestant locals and some tourists like the spectacle while others view it as reminiscent of klan rallies in the US which were signaling enterprises to let the former slaves know who was really the boss. Others still say "What? This has nothing to do with that! It's about celebrating being British. And James II was unacceptable due to him being a catholic and everything."
As a lapsed unitarian, I'd personally can't really say I understand any of it.
Anyway, I saw this episode of the hit Northern Ireland comedy show Derry Girls and it seemed to be quite a divisive thing -- even among the Derry girls themselves. Orla-- one character-- quite liked the parades. But the rest of the family thought it was reason to flee across the border and into the safety of Catholic Ireland.
I think Ireland, northern Ireland, England, Wales, Scotland-- all of it-- are quite lovely to visit. Personally, I'd pick a different day though
Happy travels!