听力原文：M: How are you going to spend this year's Thanksgiving vacation？
W: I'm going to spend it with my family. We celebrate very traditionally. We go to my grandparents' house and have a big Thanksgiving dinner. It's really the only time in the year that my whole family gets together.
M: In my family we just have a big feast, too. We don't really stop and think about how Thanksgiving Day developed, how new colonists in Massachusetts had a three-day fest to give thanks for surviving the first terrible winter and for gathering their first corn harvest the following year.
W: Did you know that the original Thanksgiving Day was celebrated in July？
M: No, I thought Thanksgiving had always been in November.
W: The first Thanksgiving was celebrated on July 30, 1623. After that, Thanksgiving was celebrated at many different times. It wasn't until 1863 that Abraham Lincoln declared the last Thursday in November as a day of Thanksgiving, and this holiday has been celebrated in November ever since.
What is the subject of this conversation？

A. How and when we celebrate Thanksgiving.
B. The traditional Thanksgiving dinner.
C. When Thanksgiving began.
D. Abraham Lincoln.