The start of the booking process depends on when tickets go on sale.
Purchase deadlines vary depending on the fare type.

For Minimum, you need to complete a booking before 23:59 (local time of your departure airport) 2 days prior to the scheduled departure date.
For Standard and Standard Plus, you need to complete a booking 1 hour before the scheduled departure time stated in the itinerary.

※To select a seat need to be completed 3 hours prior to the scheduled departure time stated in the itinerary.