At the end of the countdown the server will check if you have beat the last raid open for you.
If you have beaten it the server will open the next raid for you.
If you have not beaten it you will be stuck with the raid you could not beat.
So for example if you cannot beat the 4th raid on day 4, then you will not have access to raid 5 on day 5 because you must beat raid 4 first.
That means not beating raid 4 on day 4 makes raid 5 only available on day 6 and in turn raid 6 cannot be accessed as it resets after day 6.
That is as how I understood the introduction thread for raids.