Great question — answering as the UphillTech team:
It is strongly recommended to make a reservation. On weekends, most huts will be booked full, often including the winter rooms. Near popular places, this can also happen during the week. So it is recommended to reserve as far ahead as possible. 3 days can work out during the week, sometimes it does not. For most huts, it is possible to book a longer time in advance and cancel 2 days ahead without a cancellation fee if the weather is bad. So this is what I would recommend. In general, you are unlikely to be turned away or forced to sleep outside at high altitude. However, some huts have become stricter due to people abusing this and the overcrowding that follows. The least thing you can do...
Full answer by Manziel: https://outdoors.stackexchange.com/a/29047 (CC BY-SA 4.0).
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