"It is a special date."
Translation:Det er en speciel dato.
No it does not matter what it is replacing. When saying that "it is (insert noun)" you always use "det"
Den 2/3, det er en dato
Solen, det er en stjerne (the sun, it is a star)
If you say "it is (insert adjective) you use "den" or "det" depending on what it is. That is the reason you use den in sentence number two (den er speciel) "speciel" is an adjective
Solen, den er stor (the sun, it is a big star)
If you, like here, have both an adjective and a noun you always use "det"