Just a guess, the sap is low so the trees dry quicker, so the wood can be used quicker.  Also, the wood won't split or check as bad with lower sap levels.

It's simpler to see certain animals at night; boars for example: I've just seen 12 of them during the day, many others during nights!
Wolves... I was lucky to see 2 of them, and am waiting to find some more!
By the way I agree about the sap, but why one has to wait for the full moon? And why a lumberjack has to cut down a tree during night!?!
I love these sorts of things, rediscovering ancient traditions...  
Greetings,
Mauro.

I will not!
I am ok about the sap, and yes perhaps "the full moon" mean "cut the wood between November and Dicember": there's a month between a full moon and another!
Ancient people rulez.
Mauro.

In ancient Norwegian lore it`s well known that wood should be cut the days around full moon.  No need to do it in the dark.  
But especially do not cut it while the moon is waning, a new moon makes for better wood.
 
I`m not sure how far I trust those ancients, though
timann

Quote from wanderer on May 1^st, 2012, 3:35pm:

Sorry, my wrong, just cut the tree when the moon is increasing
timann

By the way, about the gathering interpretation: it may be, but I feel it's more adequate for veeeery ancient times, when the work of many men was needed to cut down a single tree.
The fact is, that an old woodcutter who makes his own coal in a Park inland once told me that he still cut his wood in November and December, "...when the moon is full".
He's 91 now and he told me that his father did that way too, and so did his grandfather, and there's no reason for me to doubt that this is a very ancient tradition. I'll probably go and look for European Medieval calendars, they often have paintings depicting what activity was typical for each month.
Gotta show you these!
Greetings,
Mauro.