Write SQL query to solve given problem: When did Natalie Dorris buy her first root beer?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT T2.TransactionDate FROM customers AS T1 INNER JOIN `transaction` AS T2 ON T1.CustomerID = T2.CustomerID WHERE T1.First = 'Natalie' AND T1.Last = 'Dorris' ORDER BY T2.TransactionDate LIMIT 1