请分析一下这个that从句是什么从句：

A week later as my husband and I were preparing for this year's Santa visits with my Mrs. Claus basket, we discovered a letter to Santa from a young mother named Mary, who was ill and was asking if there was a Santa and if miracles did happen that her young children would have a good Christmas because she was told by doctors that it could be her last. In it was her name, phone number and address.

附全文如下，供理解上下文：

Right before Thanksgiving, I prayed, “Lord please make me a blessing to others as Santa”.

A week later as my husband and I were preparing for this year's Santa visits with my Mrs. Claus basket, we discovered a letter to Santa from a young mother named Mary, who was ill and was asking if there was a Santa and if miracles did happen that her young children would have a good Christmas because she was told by doctors that it could be her last. In it was her name, phone number and address.

I tried to make contact with Mary, but however many times I tried, I had no luck in getting someone to answer the phone. One week before Christmas, I decided to make a call one more time.

This time a woman answered the phone. I discovered that it was Mary's mother. She first said that no one lived there by the name of Mary. I had two choices: to thank her and hang up or keep trying. I decided to continue and explained that I was Mrs. Claus and that we had found a letter written to Santa by Mary at this telephone number, expressing the desire to help her kids. She was in shock and told me that Mary had had an operation two weeks before.

The mother started crying as we continued talking. In the end, she said she would ask Mary to call me back and we would talk more. The next day, as promised, I received a phone call from Mary. We talked about her children. She gave me their names, ages, and a little about them. I told her that we wanted to fulfill her wishes and make her dream come true this Christmas to help her family. She cried and I did too as we spoke. Before hanging up, she said that she was happy that her children would have a good Christmas.

In the following days, we were busy making preparations. Much to our delight, everything was ready just before Christmas eve. We waited eagerly for daybreak.