Keep, observe, celebrate, solemnize, commemorate are comparable when they mean to pay proper attention or honor to something prescribed, obligatory, or demanded (as by one's nationality, religion, or rank), but they vary widely in their range of reference or application.
Keep and observe are closely synonymous terms, especially when they imply heed of what is prescribed or obligatory, but they differ fundamentally in their connotations.
Keep implies opposition to break, and emphasizes the idea of not neglecting or violating; thus, one keeps, rather than observes, a promise.
Observe carries such positive implications as punctiliousness in performance of required acts and rites and a spirit of respect or reverence for what one heeds or honors; when these more appropriate ideas are definitely to be suggested observe is the more appropriate term, even though keep would otherwise be possible; thus, few persons observe, rather than keep, the Sabbath in the manner of the early Puritans.
Celebrate and solemnize are also close synonyms because they may take as their objects not only a day, a season, or an occasion which for religious, political, or other significant reasons is observed with pomp and ceremony but also a ceremony or rite, usually a religious ceremony or rite, that is marked with unusual dignity and splendor.
Celebrate, however, except in certain idiomatic phrases (as celebrate the Eucharist, celebrate a marriage, celebrate Mass) in which the gravity and forms of religion are implicit, suggests demonstrations of joy or festivity (as by singing, shouting, speechmaking, and feasting).
Solemnize as applied to occasions of joy and festivity stresses their grave, ceremonious, or solemn aspects and usually suggests greater formality in observance and greater dignity and splendor of ceremony than does celebrate. The term is often specifically used of the celebrating of marriage especially with the fullest applicable religious ceremonial; thus, in the Roman Catholic Church a marriage is solemnized only when administration of the sacrament of matrimony is followed by a nuptial Mass and a special blessing.
Commemorate implies remembrance and suggests observances that tend to call to mind what the occasion (as the day, the season, or the ceremony) stands for; thus, one celebrates Christmas by religious ceremonies that commemorate the birth of Christ; the people of the United States commemorate the birth of their independence on the 4th of July; the French people commemorate the fall of the Bastille on the 14th of July.