When to book for summer holidays?

When is the latest you should be booking accommodations and airline tickets for the summer holidays? January or so?

I don't know about flights, because the fares drive me crazy, but it really depends on which dates you are planning to travel, as far as accommodation goes.

Some popular destinations can be crowded and get booked up early on summer weekends. During Obon it can be very difficult to find a place to stay, but it can be done.

For my summer trip this year, going to a couple of very popular destinations in August, I made one ryokan reservation in February, then others in March and finally June. Some ryokans won't make reservations more than a couple of months in advance, while others, or hotels, will. If you have your heart set on something specific I would do research early, but if you are flexible you don't need to be making reservations in January.