Export iCalendar file contains only prime days, like 20200109, 20200111, etc.
We deploy ical file to https://gtgteq.github.io/prime-calendar/prime-calendar.ical. If you want to import prime days, you should tell this URL to services or apps.
We use arithmoi's isPrime function for prime testing.
stack run -- range --start 20200131 --end 20200630
stack run -- term 2020 # equals to: range --start 20200101 --end 20210331
stack run -- year 2020 # equals to: range --start 20200101 --end 20201231