So I'm going on holiday in July, and I have a job that I work on Saturdays. This holiday is for 15 days, we leave on one Saturday and come back two Saturdays later. The problem is, my parents booked the holiday a bit early, before I could ask my manager for time off, and when I did fill out a holiday form the first Saturday was declined because too many people were already off that day, which is understandable because they're running a business and they need staff. Now we have a holiday that's booked, but I'm only authorised the second two Saturdays off. I warned my parents about this and they said "No matter what your manager says, you're coming anyway." but I'm not very keen on the idea of getting fired because my parents completely disregarded the fact that I potentially wouldn't be authorised the holiday.
Anyone have any ideas on what I could do?

Get a colleague to cover your shift. Cover some of theirs in return.