In late September and early October the weather can change very quickly on the 2-mile-long open space National Mall.  

Recommendations:
​Everything is expensive in Washington DC,
So make sure to bring extra Tobacco products.
A cooler for refreshments and snacks on mall.
A jacket or blanket.
A canopy for shade, Lawn, and folding chairs.
Walking Shoes there is a lot to see and most of it is done on walking tours.