Yes there are tons of shared calendar, but none perfectly suits my needs, be it functionality, ads, or UI. I want to create a very simple one for personal use between myself and my partner only. Be it just a website or an iphone app. the sync does not have to be instantaneous, probably once a day at night is fine. It does not seem possible on github pages without access to nodejs (fs). Any suggestion?
just share a google calendar, you both can add events there https://support.google.com/calendar/answer/37082?hl=en
The problem with your question is that you haven't described what your needs are. I'm sure you could get many suggestions if you let us know what features are most important to you. Obviously you don't want a calendar that is ad supported, but "functionality" and "UI" are extremely vague terms that you need to describe in more detail.