As a retiree turning 65, you should reach out to Social Security to enroll in Medicare 3 months before turning 65. Typically Medicare is effective the first of the month in which you turn 65. There are exclusions to that rule and you must contact Medicare to confirm the date you will be covered.
Your coverage under the Aetna pre-65 Medical Plan will end the end of the month prior to you turning 65. For example, if you turn 65 on August 7th, your Aetna coverage will end July 31st as your Medicare enrollment date would be August 1st.