Can I miss the last day of the internship?

Hi, I'm currently doing an internship on a construction site for my architecture degree, which starts in October. The internship runs until September 30th, but now I'm running into a problem. I just asked if there are any preparatory courses I can take before starting my studies, and there's a math course that starts on Monday, September 30th.

My question is, is it weird if I miss the very last day of my internship to attend the preparatory course? I absolutely need a certificate stating that I completed the internship by September 30th, not a single day less, because it has to be exactly six weeks, but I don't know if my boss won't include the 30th in the certificate since I won't be there. I would go to the internship anyway, even if the preparatory course wasn't so long on that day, but the course lasts exactly as long as one day of my internship.

I'll ask the boss in the next few days, but I wanted to know if it would theoretically work. Definitely, or, because I have no choice. I have to go to this preparatory course, but it's just a shame if the day doesn't count as an internship day anymore. I'd still show up there briefly and say goodbye, and then go straight to the preparatory course. That's like a normal day when I'm sick, for example; you're not there, but the day counts. Yeah, that's it. 🙂