If you are planning a trip to the U.S. in July and/or August, it is best to book as early as possible.
Half of the tourists who travel at this time book their trips more than six months in advance. This is especially true for destinations like the Grand Canyon.
While it is true that it is possible to book at the last minute, it is important to understand that your accommodation choices will be limited and flights will be more expensive than if you book early.
Outside of this peak season, it is less important to book early. In general there is greater availability, except during special events.
However, you will always save by booking as early as possible, especially on air fare and car rental.
Please note that, as each state is different (for example, California and Vermont have very different climates and are popular at different times of the year), it's a good idea to find out if you're planning your trip in the middle of peak tourist season! Visit "When should I visit the U.S.?" to learn more.