The day or evening before an event, especially a holiday or special occasion.
From Old English 'æfen' meaning evening or eventide. It contracted over time to 'eve,' and came to specifically mean the evening before a significant day.
The word 'eve' is just the Old English word for 'evening,' but we kept it as a special word mostly for holidays—so 'Christmas Eve' is literally 'Christmas evening,' which is why we don't really use 'eve' for regular Tuesdays.
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