can you get a bus from bkk to mukdahan on the same day, or would you have to book in advance because there aren't too many buses?

 

cheers

It's about a 10 hour trip.

Buses going from morning to evening or buses going from late evening through the night.

Not too many.

 

For such a long distance I would make a reservation.

If this is to cross border for visa believe most get the evening VIP bus with advance purchase leaving about 20-2100 and scheduled to arrive early morning (but believe often late).  You get bus immigration/across river/Consulate or Savannakhet bus station at same bus station in Mukdahan so no problem there (although might have to wait up to an hour).  Most report little sleep.