We weren't offered specific choices. They will offer the earliest available month (currently February 2001). When we took the tour on May 19, 2001, the earliest available was December 2000.
I asked our guide's advice on the advantages/disadvantages of the use month and stated that we would be doing most of our visits in Jan and Sep. He was glad I brought it up and allowed us to choose the June use month.
I guess you can choose any use month you'd like as long as its later than the earliest available (now Feb 2001). Since our next visit won't be until January 2002, there was no disadvantage to choosing June 2001.