What's also remarkable about this is that it's not a way to mark a specific time of year. For example, if we marked time by saying "around Christmas time" then everyone would automatically know that it's somewhere around December 25th not matter what year or time. If we were to say, it's around unleavened bread, then we would HAVE to know when unleavened bread occurs for that specific year. You could be referring to anytime between late March and late April depending on the year. So in referring to the days of unleavened bread, it was assumed by the author that people would know the time frame because they all KNEW when God's holy days were observed.
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