Hi sdnj33! Welcome to the group! All hostels HIGHLY recommend reservations, but none require them. I've never encountered one which DID NOT take reservations, however.
Just remember that summer is a peak travel season. Take that into account with the LARGE number of hostels (called "hostales" in Spanish) in the big cities which have websites increases your odds of reaching a hostel door with a sign that reads "COMPLETO" or "FULL". But sure, you'll most certainly find one which has a spare room.... you may just have to lug your luggage to 5-10 of them before you find one.
Here's a tip: To save you some time and sweat, don't go up the stairs to the reception area if the front/main/street door is open. Just push the hostels button at the portal/entrance and ask in your best Spanish "¿Hay habitaciones para esta noche?" ("Are there rooms available for tonight?") If they say NO, then thank them and go on to the next one.
Have a great time traveling around! We'd all like to hear about it when the time comes.
Saludos, MadridMan
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