On Flag Day (Friday), the manuscript will be on display at Frederick City Hall.
The document is normally kept at Maryland Historical Society headquarters in Baltimore.
On Saturday, a military procession will transport the document to Key’s grave in Mount Olivet Cemetery.
New markers will be then unveiled at the cemetery and at several other locations throughout the city.
Key was born in nearby Keymar and was a lawyer in Frederick before moving to Washington.
He penned the poem that would later become our national anthem after watching the British bombard Fort McHenry in Baltimore in September 1814.