FAR 2.101 clearly states that "day" means, unless otherwise specified, a calendar day. Unlike FAR 33.101, this definition makes no mention of the situation where the last day falls on holiday or weekend. We see nothing in FAR Part 5 or 6 on this either. Therefore, the strictest interpretation would be that the FAR says "calendar day unless otherwise specified" so the appropriate due date is the holiday. However, you should consult with your contracting officer and legal office as to the feasibility of other potential courses of action in this situation (e.g., state in a Memo for Record that for the purposes of meeting the 14-day requirement stated in FAR 6.305(a), you are using the definition of "day" defined in FAR 33.101 given that the 14th day falls on a Federal holiday).
You should also consider that, if restricting competition IAW FAR 6.302-1(c), the justification is to be posted with the solicitation; and if restricting competition IAW FAR 6.302-2, you have 30 days to post the justification (see FAR 6.305(b)).