best time to sync

[Gary Aitken]

Can someone tell me what the best time to synchronize ports is,
in terms of having a consistent ports tree?

Thought I saw something once about the end of the weekend being best
but can't find it.  In any case, is there a period during which the 
ports tree is held frozen to get a consistent build?

Tried for an answer on questions but didn't get anything helpful.

Thanks,

Gary