To send Birthday/Anniversary Greetings emails, firstly you need to create a custom field to capture the Date of Birth or Date of Anniversary. To do that, 
1. Please click the profile picture on the top right and select Account Settings -->Custom Feilds
2. Click on Add Custom field on the top right and create one of type Date.
Once the custom field is created. You need to create an Automation/Workflow.
1. Add the Scheduler Node and select the custom field that you have created under Date
2. Select either "Selected date and time" or " "Choose specific time" under Time.
3. Select the timezone
4. Under Repeat, select "Yes, when month and day matches condition"
5. Select to Repeat every year and save the changes to the node
Then add a send email node and insert the template that you have created to send.
Save the changes to the workflow/automation.
If you have created a workflow, please go to the contacts section, select all the contacts, click on bulk action, and select Execute Workflow.
If you have created automation, it will be triggered automatically based on the trigger event that you have selected.
Please check the video: