Hi, I’ve also tried to achieve it, but due to differences in grammar in different languages support of relative calendar customization is a little bit tricky, so maintainers decided not to accept my change (more details here).

However, if you’re willing to do it, you can create a fork from the original repository on the github, and then pull my changes from this branch containing my changes. Depending on what exactly you’re trying to achieve, you might need to do some further tinkering with the calendar.js, because this solution was tailored for my needs and I was focusing on full day events, but there’s a fullDayEventDateFormat config option that might help you.