The US Naval Observatory has a webpage that allows a user to enter a year and a latitude/longitude pair, and then produces a table of sunrise and sunset times for each day during that year at that latitude/longitude pair:
Sometimes, a blank entry will appear where I would expect to see a time.
For example, in the above webpage, if I scroll down to "Form B - Locations Worldwide," enter the coordinates W074 45, N42 16, leave the "time zone" blank, enter "2015" for the year, and leave all other options at their default settings, I get a blank entry for May 03. Can anyone help me to interpret this, or direct me to an appropriate reference?
There is already a symbol for when the sun does not rise and another for when the sun does not set on a particular day.