The IRS is counting in days not business days. Unlike other tax filings, these are "calendar day" deadlines. If your 45th day is Christmas, your identification is due by midnight on Christmas. There are zero extensions unless you are in a Presidentially declared disaster area.
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No. After midnight on the 45th day, your list is "locked." You must close on one (or more) of the properties on that list. If they all fall through on Day 46, your exchange fails, and the taxes become due.