so unaccommodating!

Aesthetic elements spa promote a service on Groupon. A full day spa treatments. This was purchased by me and a few days later I called to make an appointment to redeem my voucher. Not only could I not get through to the spa as it was continuously engaged but when I eventually got through I was told that the voucher could only be redeemed on a week day as weekend days were fully booked. Due to work commitments I explained that I cannot take time off work to redeem the voucher during the week. The person I spoke to not only told me that I can request a refund from Groupon but that they had 600 purchases of the treatment. Why is that my problem? Why did they sell 600 if they couldn't accommodate so many people. Furthermore I get told that in the terms and conditions it was subject to availability. They were offering a service not an item that could be sold out which is then subject to availability. I think it's appalling to promote a service and then not accommodate the client on the dates that suited them.