The effective date of a policy could also be referred to as the start date for coverage, it's when you're policy goes into effect. Exactly when this date falls is determined by the type of policy you're buying.
If you're purchasing a comprehensive policy for all inclusive coverage, then the effective date of coverage is one minute after midnight the day after you purchase the policy. So if you bought your plan today, then the coverage for trip cancellation will go into effect one minute after midnight tonight. The other benefits (interruption, delay, baggage, medical, etc.) all go into effect when you actually depart on your trip, since you wouldn't need them until then.
For any other policy like medical, evacuation, or accidental death, the coverage will commence on the scheduled date of departure. One difference here is with the multi-trip policies. With these plans, the effective date is whenever you choose to have the calendar year of coverage actually begin, and then coverage will go into effect every time you leave your home country.