From the Columbia Guide to Standard American English:
Special and especial are adjectives, special much more frequently used: She’s a very special sort of person. Special appears increasingly in stereotypical phrases such as special agent and special delivery, including a growing number of euphemisms such as special education, which stress the idea that something special is “designed or intended for a particular purpose”; hence “unusual.” Especial is more Formal in many uses, perhaps because it occurs relatively infrequently. The adverbs specially and especially are distributed somewhat differently: uses such as specially planned, specially constructed, and specially created are very common; especially only rarely occurs in them. Otherwise, the adverbs are semantically interchangeable, although especially is a bit more formal in some contexts, where it contrasts with the aphetic specially: We were especially pleased you’d decided to join.
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