An annual travel insurance starts from the day you purchase this policy and people usually and covers you for the next one year, so people usually try to make the first day of your travel as the first day of the travel insurance cover. But there is one point that always need to be taken care of that in case of cancellation of your trip you will not be covered under this policy till your policy begins. My suggestion is to book your policy from the day you plan and book your holiday. And if you are planning a special long holiday then you should also check out with your company that it provides cover to the cost of cancellation of the entire trip.