The Collaborative International Dictionary of English v.0.48:

Observance \Ob*serv"ance\, n. [F. observance, L. observantia.
   See Observant.]
   1. The act or practice of observing or noticing with
      attention; a heeding or keeping with care; performance; --
      usually with a sense of strictness and fidelity; as, the
      observance of the Sabbath is general; the strict
      observance of duties.
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            It is a custom
            More honored in the breach than the observance.
                                                  --Shak.
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   2. An act, ceremony, or rite, as of worship or respect;
      especially, a customary act or service of attention; a
      form; a practice; a rite; a custom.
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            At dances
            These young folk kept their observances. --Chaucer.
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            Use all the observance of civility.   --Shak.
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            Some represent to themselves the whole of religion
            as consisting in a few easy observances. --Rogers.
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            O I that wasted time to tend upon her,
            To compass her with sweet observances! --Tennyson.
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   3. Servile attention; sycophancy. [Obs.]
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            Salads and flesh, such as their haste could get,
            Served with observance.               --Chapman.
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            This is not atheism,
            But court observance.                 --Beau. & Fl.
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   Syn: Observance, Observation. These words are
        discriminated by the two distinct senses of observe. To
        observe means (1) to keep strictly; as, to observe a
        fast day, and hence, observance denotes the keeping or
        heeding with strictness; (2) to consider attentively, or
        to remark; and hence, observation denotes either the act
        of observing, or some remark made as the result thereof.
        We do not say the observation of Sunday, though the word
        was formerly so used. The Pharisees were curious in
        external observances; the astronomers are curious in
        celestial observations.
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              Love rigid honesty,
              And strict observance of impartial laws.
                                                  --Roscommon.
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WordNet (r) 3.0 (2006):

observance
    n 1: the act of observing; taking a patient look [syn:
         observation, observance, watching]
    2: a formal event performed on a special occasion; "a ceremony
       commemorating Pearl Harbor" [syn: ceremony, ceremonial,
       ceremonial occasion, observance]
    3: the act of noticing or paying attention; "he escaped the
       notice of the police" [syn: notice, observation,
       observance]
    4: conformity with law or custom or practice etc. [syn:
       honoring, observance] [ant: nonobservance]

Moby Thesaurus II by Grady Ward, 1.0:

247 Moby Thesaurus words for "observance":
   accommodation, accord, accordance, acquiescence, adaptation,
   adaption, adherence, adhering, adjustment, adoration, advertence,
   advertency, agreement, alertness, allegiance, anniversaries,
   assiduity, assiduousness, attention, attention span, attentiveness,
   awareness, baccalaureate service, bon ton, care, carrying out,
   celebrating, celebration, ceremonial, ceremony, churchgoing,
   cognizance, commemoration, commencement, completion, compliance,
   complying, concentration, conformance,
   conformation other-direction, conforming, conformity, congruity,
   consciousness, consideration, consistency, consuetude, convention,
   conventionality, convocation, correspondence, cultism,
   custodianship, custody, custom, devotedness, devotion, devoutness,
   diligence, discharge, dressing ship, duteousness, dutifulness,
   duty, eagle eye, ear, earnestness, effectuation, empty formality,
   enforcement, espial, espionage, established way, etiquette,
   examination, execution, exercise, exercises, faith, faithfulness,
   fanfare, fanfaronade, fashion, fealty, festivity, flexibility,
   flourish of trumpets, folkway, form, form of worship, formal,
   formality, formula, formulary, fulfillment, function, graduation,
   graduation exercises, guard, guardedness, guardianship, habit,
   harmony, heed, heedfulness, heeding, holiday, holy rite, homage,
   inaugural, inauguration, initiation, inspection, institution,
   intentiveness, intentness, invigilation, jubilee, keeping, line,
   liturgy, looking, lookout, love of God, loyalty, malleability,
   manner, manners, mark, marking the occasion, memorialization,
   memory, mind, mindfulness, mode of worship, monitoring, mores,
   mummery, mystery, note, notice, obedience, obediency, obeying,
   observation, observing, office, order of worship, ordinance,
   orthodoxy, ovation, peeled eye, performance, pietism, piety,
   piousness, pliancy, practice, praxis, prescribed form,
   prescription, proctoring, proper thing, prosecution, prudence,
   qui vive, recognition, reconcilement, reconciliation, regard,
   regardfulness, rejoicing, religion, religionism,
   religious ceremony, religious rites, religiousness, remark,
   remembrance, respect, respecting, revel, reverence, rite,
   rite de passage, rite of passage, ritual, ritual observance,
   rituality, sacrament, sacramental, salute, salvo, scrutiny,
   service, servility, servitium, sharp eye, social convention,
   solemn observance, solemnity, solemnization, spying,
   standard behavior, standard usage, standing custom, stewardship,
   strictness, submission, submissiveness, suit and service,
   suit service, surveillance, testimonial, testimonial banquet,
   testimonial dinner, theism, thought, time-honored practice, toast,
   tradition, traditionalism, transaction, tribute, triumph,
   uniformity, usage, use, veneration, viewing, vigil, vigilance,
   wariness, watch, watch and ward, watchful eye, watchfulness,
   watching, way, weather eye, what is done, willingness, witnessing,
   wont, wonting, worship, worshipfulness