Count the number of unique visits (date for each customer)

I have a table of ticket sales, showing each individual ticket sold and the date of the event. In a related table (Customers), I have a list of CRM ID's and names.

A customer buys 5 x tickets to one event on one date, then another 3 tickets to an events on another date. I need to have either a measure or a column in a table to show how many distinct dates a customer has visited. In this case, this would be 2 visits. I'm not sure if this should be a measure, or a column in the Customers table.

Plus, I need help on how to write the formula please!

thanks!