Important.
particular, functional
detailed, special, peculiar
odd, special, unusual
To Emit
Preferred
characteristic, particular, definite, especial, exact, special
Try... unique, unusual, creative, one of a kind
A red letter day is a day of special significance. You might say a celebration. You could also call it an important day, or a special day. Another term you might hear is an observance.
special case, freak, anomaly, inconsistency, deviation, oddity, peculiarity, irregularity
An information sheet or bulletin, or a report circulated periodically to a special interest group.
different, atypical, unfamiliar, uncommon, unique, special, rare, odd, extraordinary, amazing
Instead of using the word challenging you can use the word gifted, special, special gifted.