Autumn in Tokyo means crisp weather, changing leaves at Meiji Jingu, and incredible street food in neighborhoods like Omoide Yokocho. Booking about four months out gives you the sweet spot for flight availability and lets you lock in mid-range hotels before peak foliage pricing kicks in.
A typical US traveler spends about $2800 on this trip. Since that is a chunk of change to drop all at once, you might look at ways to spread out the payments so your bank account does not take a sudden hit.
Remember that the intro window is a hard deadline, not free money. If any part of that $2800 balance is still sitting there when the 15 months run out, standard interest kicks in on the whole original amount.