The hostels get very, very busy during the summer months and it is certainly advisable to book as early as possible in order to get a cheap bed.
Also, be aware that if you have not warned the hostel that you will be making a late check-in (and by late check-in, they mean any time after 6pm) your reservation may be cancelled and given to another person. So if there's any chance of delay, send an email or call in advance of your arrival.
(The alternative is, if you missed out on point one, to hang around hostel receptions at about 6pm to see if they are yet ready to make some cancellations).
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