Guild Wars 2

Does this mean it is ongoing or is it a one time only event?

If I don’t manage to get to the game today, will I still be able to complete it?

I

The only way to miss this stuff is to avoid the event until it ends.

I think with the exception of the weekend fireworks, all the content will be available until the end of the event. They are just slowly releasing the content over the course of the week.