Dream, it is very unlikely they will do it. The only way to do would be to talk to them ... a letter is unlikely to give you a confirmation soon enough - a few days in mail, a few days deciding, then holidays and before you know it you are wondering why there is no response and it is already interview time.
The reason they will not do it because they can hide behind the process of processing applications in order. Most likely they will push the date out if you say you are not available on Jan 15.