You have a couple options.
Our student affairs office will hold certain LORs until ERAS opens, so you can find out if your school will do that.
Alternatively, you could ask for a letter, so they can write about your attributes while you're still fresh in their mind, but request that they hold onto it until ERAS opens next year. When you get into ERAS, you can get back in contact with them, send your personal statement and updated CV as well as the letter request form, and get them to upload it.