Federal law
Recurring national observances include Memorial Day, Peace Officers Memorial Day, Patriot Day, and Pearl Harbor Remembrance Day.
U.S. flag code
When Should the Flag Be at Half Staff?
Lower the flag when federal law or a valid presidential or gubernatorial order calls for it.
Presidential order
The President may order flags at federal buildings and across the nation to half-staff after the death of officials or during national mourning.
Governor's order
A governor may order flags to half-staff within that state. These orders do not automatically apply nationwide.
How to raise and lower the flag
Hoist the flag briskly to the peak before lowering it to half-staff. Before lowering it for the day, raise it to the peak again.
Read 4 U.S.C. § 7 ↗