THOMASTON – The General Henry Knox Museum, housed in the Montpelier mansion, has more to celebrate this month than the famous war soldier’s 251st birthday.

Last week, the Friends of Montpelier, which maintains and operates the facility, purchased a 5-acre parcel behind the museum, which was owned by Montpelier 70 years ago. The acquisition doubles its land holdings and enables the museum to expand programs, museum Director Lucy Ebbert said Friday.

On Saturday, July 21, Maj. Gen. Henry Knox’s birthday will be celebrated at the museum, located at the intersection of Routes 1 and 131. Also celebrated will be the 75th anniversary of the Knox Masonic Lodge of Boston.

In 1931, the land behind the museum was owned by Montpelier but was sold for financial reasons, Ebbert said, noting that Dorothy Jameson of Thomaston sold the property to the museum Tuesday for an undisclosed price.

The Friends of Montpelier is uncertain as to what will be done with the land, Ebbert said. The group is leaning toward someday having a learning or educational center there, she said.

“We want to be more of a living museum,” Ebbert said, adding that the first step was to acquire the land.

The celebration of Knox’s birthday begins at 9:30 a.m. with a march from the Masonic Lodge on Main Street to the town cemetery for a wreath-laying ceremony at his grave and a musket-firing salute. From there, an open house will be held at the Thomaston Historical Society building on Knox Street.

A noon flyover at the museum by a Brunswick Naval Air Station aircraft honors the Knox Lodge and will be followed by the raising of a 13-star “Betsy Ross” flag and music by a costumed fife and drum corps.

This year marks the first time that artillery men in uniform will fire the Hope Cannon, a replica of a Revolutionary War artillery piece used to greet Old Ironsides in Boston Harbor from 1986 to 2001.

Under the direction of Gen. George Washington, Knox was instrumental in hauling 59 cannons from Fort Ticonderoga to Boston in 1775 during the siege of the city by the British. The invaders were forced out in March 1776, because of his effort.

The Friends of Montpelier will accept the cannon, donated by the Knox Masonic Lodge of Boston, as a symbol of Knox’s important position as head of artillery during the Revolutionary War.

Tickets for the 12:30 p.m. chicken barbecue can be purchased in advance or at the event. Call 354-8062.