Flexibits has issued Fantastical 2.3.8, a small but important update to the popular calendar app. Starting 15 June 2017, iCloud will require two-factor authentication and an app-specific password, and Fantastical will stop syncing your iCloud accounts if you do not take action. This update contains instructions for setting up two-factor authentication and generating an app-specific password. ($49.99 new from Flexibits and the Mac App Store, free update, 14.0 MB, release notes,
I'm a bit confused about how "This update contains instructions for setting up two-factor authentication...". I installed the update, and didn't see any instructions. Nor did I find much enlightenment on the Flexibits website, beyond what appears in this TidBITS article.
I think this page has the information you want:
It's not very hard, so it might have seemed like there should be more information than there is.