It's indeed an annual event. Decades ago they run the top 100, then it became top 1000, and now the top 2000.

You calculations are about right: it started 7 days before year end

Have some links for you... The radio jockies are ofcourse all talking Dutch... but maybe you can understand some if it 😉

The top 2000 list: https://www.nporadio2.nl/top2000
All one hour episodes to watch: https://www.nporadio2.nl/top2000/terugkijken
The radio stations can be watched live (top2000 has ended though): https://www.nporadio2.nl/live
And listened to at: https://www.nporadio2.nl/online-radio-luisteren

I do hope all these streams can be watched outside NL; When not, let me know and I'll try and figure out how you can connect.