To book or not to book Youth Hostels?

Hi! I'm currently planning a trip to Japan next winter and I'll try to stay mainly in Youth Hostels... But the problem is that I don't know exactly when I'll be in specific cities, so I don't really want to book in advance. Can I book only like 1-2-3-4-5... days in advance?

Thank you!

If they have space . . .

I would definitely book your first nights in Tokyo or wherever when arriving. After that, you can check online for some hostels, and for others you can have the hostel you're staying at check the next place you're heading.

One problem is hostel size as some are fairly small, and one group will fill the place. If you don't limit yourself to just hostels, you can usually find cheap accommodation somewhere when there aren't any rooms in the YH.

You can also just head to the next place that has a bed if your trip is loosely enough planned.

Since you mentioned winter, areas without ski slopes or beaches shouldn't be too crowded.