Yes, you may wait until you arrive and check the weather before deciding which day to go up to Jungfraujoch, and then make a reservation at the ticket counter on site.

! Reminder:

Please note that during peak season, especially from May to October, popular time slots can sell out quickly. You may not be able to travel at your preferred time and may need to take a different departure instead.

It is recommended to select a tentative visit date and make a reservation in advance. If the weather turns out to be very poor on your travel day, you may rebook another time slot by paying a new reservation fee. The original reservation fee is non-refundable, but your train ticket itself will remain valid, meaning you would only lose the reservation fee.