Most often with hostels (called "hostales" in Spanish or "hostals" in Catalán) they'll have a button at the "portal" or doorway which you can buzz and the owner will call down to see who it is. Just ask, "¿Hay habitaciones?" (Are there any rooms available?) and they'll either say "Sí. Sube" (Yes, come up") and they'll buzz you in the front door and then you'll go up one or two (or three) flights of stairs where they'll receive you.
OR you'll buzz the button and they may say "No. Está completo" (No. We're full).
Whatever you decide, I recommend visiting a number their websites of those in areas you're considering and at least then you'll have an idea of where they are, their prices, and how it looks on the inside before you even get there. This can save you A LOT of time. Check out the MadridMan list
HERE of 100+ hostels in Madrid.
If your first night there is the Thursday the 30th, TOMORROW NIGHT (maybe this message is too late for you), you'll probably be fine but it's more difficult to find a room over the weekend. In fact, you may find one with a room available for Thursday night but that same hostel may be full over the weekend so you MAY have to move to another one. Wow.. good luck.
Saludos, MadridMan
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