Once you have purchased the air tickets and/or made hotel and other bookings, you have already spent a large chunk of your trip budget. It is best to buy your travel insurance policy soon after you buy your tickets and make bookings as many policies offer trip cancellation and interruption benefit wherein the Company will pay this benefit up
to the Sum Insured as specified in the policy for trips that are canceled before the scheduled departure date due to certain specified reasons . The Company will reimburse for the forfeited, nonrefundable prepaid payments, made prior to the Insured Person’s departure date.

Before departure you can even cancel your policy by paying a minimal charge or even in some companies, no charge at all. So, there is no harm in buying your policy. Don't wait till the last minute to buy your travel insurance as sometimes, server issues or payment issues might also be there due to which you may have to buy a plan different from the one you have selected.

Note: Please check plan benefits while buying travel insurance whether trip cancellation is covered or not. Also refer policy wording to check the circumstances under which this benefit is payable.