keep: suggests a customary or wonted noticing without anything untoward or inappropriate: ~ THE Sabbath.
observe: suggests marking of occasion by ceremonious performance of required acts and rituals: today not all holidays are ~ nationally.
celebrate: suggests acknowledge of an occasion by festivity and indulgence: they traditionally ~ Thanksgiving with a huge dinner.
commemerate: suggests marking of occasion by observances that remind one of origin and significance ~ Memorial Day with laying of wreaths.
keep只是习俗性注意，observe举行仪式，celebrate庆祝欢宴，commemorate记住起源意义的仪式。
记忆方法： 1) 通常说的过节庆祝，实际是纪念或致敬，这组词的基本意思是"纪念"。
2) 最基本的keep可能只是习惯性注意，其他3个都有仪式或庆祝，都解释为纪念某一时刻occasion。
3) 首字母记忆KCOC，把COC联想为Code Of Conduct行为规范，K为Keep，KCOC就是"纪念行为规范"。
4）纪念的意思是对事迹表达敬意和怀念，通常在特定日期场合mean to notice or honor a day, occasion, or deed.
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