Cheapest route from Bangkok to Koh Chang in new years

I am arriving in Bangkok on the 26th of December as a couple, arriving early in the morning in Don Mueang airport. I’m planning to go to Koh Chang on the 28th of December to the 30th of December.

I have done some research about how to get from Bangkok to Ko Chang and I decided to use the 999 bus to Koh Chang pier as it is the cheapest. I’m planning to go directly to Ekkamai bus station from the airport the day I arrive and buy the return tickets, so I’ll buy the tickets two days in advance but I heard it can get busy during the new years.

Is buying two tickets two days in advance enough time to get me a seat in the bus during new years?

Any advice or recommendations will be much appreciated! Thanks!

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