It's up to you when you decide to purchase your travel insurance, however most people take it out as soon as they've booked their trip, in order to cover them for cancellation.
If you are looking for a single trip policy, you're covered for cancellation from the day of purchase, and you need to input specific travel dates to indicate when you're away.
With an annual multi-trip however, your policy is live for 365 days, covering you from the date you decide. This means that your policy doesn't need to begin on the day of purchase if you don't want/need it to.
We would always recommend making sure you select the correct start date to benefit from Cancellation cover.