Visa from Kathmandu to Tibet

Hi, I am wanting to do a tour to Tibet. I know that I need to get a group visa from a Travel Agent. I will be in Kathmandu in the afternoon of the 7th. My Agent has told me your office is closed the 9th and 10th so this will not allow enough time to obtain the visa for China for a tour that starts on the 12th. I am wondering as I am in Kathmandu on the 22nd September can I start to organise my visa then and does that mean that you or the travel agent would keep my passport until I return. Thank you.