Why do some dates not have any data on the Federal Reserve historical currency data?

Example: https://www.federalreserve.gov/releases/h10/hist/dat00_al.htm

Some of the days, such as 12-Nov-01, say ND instead of a price. I assume that this means "No Data", but it’s not explained anywhere.

Why would some days not have this data? And it’s quite a few days, too, if you scroll down the huge table.

If I’m using this to look up how much money was worth for a certain date, should I instead pick the next or previous day? Or maybe some sort of average calculation based on nearby days?

Primarily, I’m asking why some days lack this data to begin with.