Hi there @kissed1
Welcome to the Klaviyo Community and thank you for your question!
To be able to use the Anniversary Flow (there is a great one pre-built in Klaviyo), you will need to ensure that customer profiles are being updated (tagged) with their first purchase date with a Profile Property. This step can happen within the flow.
Then you can set the flow to send the email communications 365 days later.
There is some great information available here: https://help.klaviyo.com/hc/en-us/articles/360001768432#update-profile-property-examples
And some great use cases detailed here to help put it into perspective: https://help.klaviyo.com/hc/en-us/articles/360050420371
I hope this information helps!
Kylie