"Eclipse day" refers to a day when a celestial event called an eclipse happens. It occurs when one astronomical object obscures or covers another. This term is specifically used to describe the day when an eclipse is happening. Full definition
You look absolutely adorable and wishing you a happy eclipse day! (rosyoutlookblog.com)
Then on eclipse day — Monday, August 21, 2017 — we head out from Nashville in the morning hours to our specially selected viewing site in a small Tennessee town, where you enjoy a uniquely American eclipse experience. (discovermagazine.com)
Overall, the regional cloud pattern bodes well for eclipse day, as the approaching shadow of the Moon will cause temperatures to fall, and therefore much of the convective cloud will dissipate. (discovermagazine.com)