I'm wondering if in Harry Potter the Yule Ball occurs every year, or if there was only one because of the Triwizard tournament?
It's only part of the Triwizard Tournament.
‘Now Potter and Weasley have been kind enough to act their age,’ said Professor McGonagall, with an angry look at the pair of them as the head of Harry’s haddock drooped and fell silently to the floor – Ron’s parrot’s beak had severed it moments before – ‘I have something to say to you all.
‘The Yule Ball is approaching – a traditional part of the Triwizard Tournament and an opportunity for us to socialise with our foreign guests. Now, the ball will be open only to fourth-years and above – although you may invite a younger student if you wish –’
Harry Potter and the Goblet of Fire, via Pottermore
It's only during the Triwizard tournament. If there were any other kind of annual ball they would most probably already have dress robes.