Salisbury's wolf's-head hat was part of his mountain man from the 1700s dress.
As many as 6,000 people are expected to attend the event, which features historical demonstrations and more than 150 traders with wares that could have been found 200 years ago.
The park also is hosting about 1,500 period campers, who dress in clothes from the 18th and early 19th centuries.
The period campers such as Salisbury sleep near the fort in tents or tepees.
The event will be open to the public today and Saturday from 9 a.m. to 5 p.m., and Sunday from 9 a.m. to 3 p.m. Park admission is $3 per person, except for children under 6 who get in free.
The historical nature of the park and the quality of the traders help make the event so popular, said Davis, a member of the Patuxents, who organize the annual event.
The traders, called sutlers, come from all over the country and sell everything a 1700s-era citizen could need, from muzzleloading rifles to animal hides and moccasins.
"If you want 18th century reproductions this is the place to come," said Mark Lore, of Winchester, Va.
Lore and his wife Sandy were among the few people at the park Thursday not dressed in old-style clothes. But Sandy Lore was buying some. She bought socks, a white hat and a nightgown-looking garment she called a shift, which will be part of an 18th century Quaker costume she is putting together.
The event brings some attention to the park and educates the public about life in the 18th century, said Ralph Young, the park manager.
Fort Frederick was first built in 1756 on what was then considered the frontier, he said.