Also I was thinking of going just for a week for the experience, but if I understand correctly that would slow the metabolism and you would still need to eat carefully for a month or so afterward; again cutting into my holiday... Unless someone has good reason for me to do a short 7 day fast now otherwise I will just continue intermittent fasting.

Hi Bio :<),

A week long fast would slow your metabolism somewhat (likely about 10%) but within a couple weeks of breaking you would be right to normal. Careful eating following a 7 day fast would typically have you right back to full normal eating within about 7 days of breaking. 

A 7 day fast at this time would provide you with fairly substantial benefit and you would easily be in a good position to safely face your holiday eating a month from now.  You mentioned that you have been mostly low fat raw vegan for 20 months, but you expect during the holidays to not be eating this way?  Either way, your digestive system should certainly be fully functioning by then.  If it was me, i would even feel quite confident going 14 days at this time and being completely ready to enjoy the holidays in a month.

Oh I've a question about that if you don't mind. After only fasting for 24hrs I seem to break into headaches. I thought this was a sign of ketosis which I know happens after 72hrs. Could the intermittent fasting I've been doing be allowing my body to convert to ketosis earlier than most?

I agree with other posters that this sounds like classic detox which seems to be true in spite of your mostly lfrv lifestyle in the past 20 months.  In attempting to get to the bottom of this mystery, if it were me I would consider how pesticide/herbicide free is my produce, how pure is my water, how pure is the air that i breathe, and are there any other factors whereby toxins are entering my body. 

Even if you were somehow going into ketosis before the 72 hours through your special circumstances, i  have never known ketosis itself to cause headaches, neither experientially, nor from anything i have read... and it is my understanding that most headaches will come and go within the first 3 days when detox is the most concentrated and before ketosis normally sets in.  Somebody please correct me if the literature states otherwise.

Best wishes to you.