When do I tell my employers that I'm leaving?

So basically I joined this start-up as an intern in February and at the same time also had an offer from a university for master's degree. I enjoyed working and ultimately requested for a deferral from the University, so I will be joining next year ie, September 2020.

I'm involved in a lot of things in the company and I'm sure whenever I leave, I'll have to explain a lot of things to people so that they can take over the work I do. People in our company are very open about things so I was wondering, do I tell them now I'll be leaving next year or just hand my resignation next year whenever I have to leave?

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