How would you interpret "due on or before Monday 11:59"....?

I don't typically leave things to the last minute, but this week I unfortunately had to do just that. :/

I just submitted my assignments because I interpreted it to mean that I had until 11:59 tonight?

But after reading it a couple of times, I'm starting to panic and overthink. What if the instructor meant Sunday night? Idk.

What are your thoughts? Am I doomed here?