There are lots of cute Velvet dresses which would fit that bill, such as this colorblocked number. The colors and inherent stretchiness of the jersey fabric = perfect for Thanksgiving.
The more modest cut of the Rachel Pally dress would make it ideal for the office Christmas Party - throw on some big statement jewels and you will be just as festive as everyone else wearing their red sweaters and a heck of a lot chicer.
Even though I've been wary about maxi dresses in the winter, surprisingly my favorite pick for the biggest holiday celebration I attend, Christmas Eve, is this Odylyne maxi dress. The metallic fabric makes it perfect for an evening affair, and the asymmetrical hemline keeps it from feeling too formal.
Of course nothing would make shopping for the perfect party dress better than having $100 to spend at Shopbop, so go on, start entering!