Normally, the mail will only come at 9am and 5pm daily. However, there is a way to make the mail come whenever you want. When you order an item from the catalog, or when you or one of your villagers sends a letter, it becomes "pending to be sent" at the post office. However, the post office can only hold ten letters as "pending" at a time. If more than ten letter are sent, all the mail will come. So, in order to cause the mail to come whenever you want, simply send letters to one of your villagers or another character in your house until there are more than ten total letters waiting to be sent. This is a great way to get items from the catalog whenever you want.