mid-15c., "to perform publicly with appropriate rites," originally of the Mass, from Latin celebratus "much-frequented; kept solemn; famous," past participle of celebrare "assemble to honor," also "to publish; sing praises of; practice often," originally "to frequent in great numbers," from celeber "frequented, populous, crowded;" with transferred senses of "well-attended; famous; often-repeated." Its etymology is unknown.
General sense of "commemorate or honor with demonstrations of joy" is from 1550s; formerly it also could be with demonstrations of sorrow or regret. Meaning "make widely known, praise, glorify" is from 1610s. Related: Celebrated; celebrating.