The most popular iCalendar (RFC 5545) class library for .NET.
$ dotnet add package Ical.NetiCal.NET is an iCalendar (RFC 5545) class library for .NET aimed at providing RFC 5545 compliance, while providing full compatibility with popular calendaring applications and libraries.

iCal.NET is available as a nuget package.
There's a guide just for you: Migrating from dday.ical
The wiki contains several pages of examples of common ical.net usage scenarios.
ical.net uses semantic versioning. In a nutshell:
Given a version number MAJOR.MINOR.PATCH, increment the:
- MAJOR version when you make incompatible API changes,
- MINOR version when you add functionality in a backwards-compatible manner, and
- PATCH version when you make backwards-compatible bug fixes.
We ask and encourage you to contribute back to the project. This is especially true if you are using the library in a commercial product.
Questions asked in the discussion area are open to the community or experienced users to answer. Give maintainers a helping hand by answering questions whenever you can.
Remember that keeping ical.net up is something ical.net maintainers and many contributors do in their spare time.
In case you need it, Rian Stockbower may offer paid support and bugfixes. A few basic rules to consider when asking for this kind of support:
iCal.NET logo adapted from Love Calendar By Sergey Demushkin, RU