If you want to catch the cherry blossoms, then you should plan on being in Seoul around the second week of April (around April 9-10). That would be safest. However, please be advised that the cherry blossoms are largely dependent on the weather so it’s no guarantee that they’ll be blooming then. If it gets warmer earlier, then they’ll bloom earlier. If the weather stays cool, then they’ll bloom later. Such is the difficulty of planning your trip around the cherry blossoms. Be sure to bookmark the Visit Korea website for updates on the festival.