"it is a special date" requires the copula, because you are linking a pronoun ("it") and a noun ("date"). The presence of the attributive adjective "special" isn't relevant to the requirement to use the copula. Is dáta speisialta é.
"the date is special" is not a copula, you just have a noun ("date"") and a predicative adjective "special" - tá an dáta speisialta.
Completely different words - spéis, with a fada, means a fondness or an interest. It gives rise to the adjective spéisiúil.
speisialta, meaning "special", doesn't have a fada, and therefore does not sound like spéisiúil. (speisialta sounds a lot like "special").
Fadas matter. A vowel with a fada is a completely different letter from the same vowel without a fada.