It is very easy, thirty hours before the flight, in this case, the line disappeared and was replaced by the invitation to check in now.
So it was not about it being on a national holiday, it was just the flight not yet being open to be checked in.
Of course, not all KLM flights open for checking in 30 hours before departure, in the case it is a different length of time before checking in is allowed, the message change will be on the time checking in opens.
And if your situation is not as straight forward, it might be a different case, you may not be able to check-in online at all.
KLM, if you read this, maybe you can add that one more word to the line, the word 'yet'.