red-letter day(n = noun.time) - a memorably happy or noteworthy day (from the custom of marking holy days in red on church calendars);
is a kind of day

N  importance, consequence, moment, prominence, consideration, mark, materialness, import, significance, concern, emphasis, interest, greatness, superiority, notability, weight, value, usefulness, gravity, seriousness, solemnity, no joke, no laughing matter, pressure, urgency, stress, matter of life and death, memorabilia, notabilia, great doings, red-letter day, great thing, great point, main chance, the be all and the end all, cardinal point, substance, gist, sum and substance, gravamen, head and front, important part, principal part, prominent part, essential part, half the battle, sine qua non, breath of one's nostrils, cream, salt, core, kernel, heart, nucleus, keynote, keystone, corner stone, trump card, salient points, top sawyer, first fiddle, prima donna, chief, triton among the minnows, 'it', important, of importance, momentous, material, to the point, not to be overlooked, not to be despised, not to be sneezed at, egregious, weighty, of note, notable, prominent, salient, signal, memorable, remarkable, unforgettable, worthy of remark, worthy of notice, never to be forgotten, stirring, eventful, grave, serious, earnest, noble, grand, solemn, impressive, commanding, imposing, urgent, pressing, critical, instant, paramount, essential, vital, all-absorbing, radical, cardinal, chief, main, prime, primary, principal, leading, capital, foremost, overruling, of vital importance in the front rank, first-rate, superior, considerable, marked, rare, significant, telling, trenchant, emphatic, pregnant, tanti, materially, in the main, above all, kat' exochin, par excellence, to crown all, to beat all, expende Hannibalem!.

N  repose, rest, silken repose, sleep, relaxation, breathing time, halt, stay, pause, respite, day of rest, dies non, Sabbath, Lord's day, holiday, red-letter day, vacation, recess, reposing, relaxed, unstrained, unstressed, at rest, The best of men have ever loved repose, to repair our nature with comforting repose.

N  celebration, solemnization, jubilee, commemoration, ovation, paean, triumph, jubilation, ceremony (rite), holiday, fiesta, zarabanda, revelry, feast (amusement), china anniversary, diamond anniversary, golden anniversary, silver anniversary, tin anniversary, china jubilee, diamond jubilee, golden jubilee, silver jubilee, tin jubilee, china wedding, diamond wedding, golden wedding, silver wedding, tin wedding, triumphal arch, bonfire, salute, salvo, salvo of artillery, feu de joie, flourish of trumpets, fanfare, colors flying, illuminations, inauguration, installation, presentation, coronation, Lord Mayor's show, harvest-home, red-letter day, trophy, Te Deum, fete, holiday, Forefathers' Day, celebrating, commemorative, celebrated, immortal, in honor of, in commemoration of, Int, hail!, all hail!, io paean, io triumphe!, see the conquering hero comes!.

admission day, all day long, all fools' day, all saints' day, all souls' day, american indian day, annunciation day, april fools' day, arbor day, armed forces day, armistice day, ascension day, bastille day, bissextile day, boxing day, break of day, break of the day, by the day, calendar day, call it a day, candlemas day, childermas day, christian holy day, christmas day, citizenship day, civil day, clarence day, clarence shepard day jr., class day, columbus day, commencement day, commonwealth day, day, day after day, day bed, day blindness, day boarder, day book, day by day, day camp, day care, day care center, day game, day in and day out, day in day out, day jessamine, day laborer, day labourer, day lily, day nursery, day of atonement, day of judgement, day of judgment, day of reckoning, day of remembrance, day of rest, day of the month, day of the week, day off, day return, day school, day shift, day watch, decoration day, degree day, discovery day, dominion day, each day, easter day, election day, ember day, empire day, every day, fast day, father's day, feast day, fete day, field day, flag day, forever and a day, future day, gain the day, good day, groundhog day, guy fawkes day, high holy day, holy day, holy day of obligation, holy innocents' day, inauguration day, independence day, innocents' day, jewish holy day, judgement day, judgment day, labor day, lady day, lammas day, last day, leap day, lord's day, lunar day, major fast day, market day, martin luther king day, may day, mean solar day, memorial day, michaelmas day, midsummer day, midsummer's day, minor fast day, mother's day, naked as the day one was born, naked as the day you were born, name day, natal day, new year's day, night and day, order of the day, pan american day, pancake day, patriot's day, patriots' day, pioneers' day, polling day, poppy day, present day, presidents' day, quarter day, rag day, rainy day, red-letter, remembrance day, rest day, robert e lee day, rogation day, saint patrick's day, saint valentine's day, saint's day, school day, seize the day, sidereal day, six day war, solar day, speech day, st john's day, st martin's day, st patrick's day, st valentine's day, tag day, term day, texas independence day, thanksgiving day, three kings' day, time of day, transfiguration day, twelfth day, united nations day, unlucky day, v-e day, v-j day, valentine day, valentine's day, veterans day, veterans' day, victoria day, victory day, washing day, wedding day, work day, working day