As soon as you're ready and whatever suits you best logistically.
People tend to try for Fridays if they are trying to dovetail a sale and purchase so that they have the weekend to get the new house sorted. If it all goes pearshaped, getting the movers again on Monday can be a problem.
If it was my choice, no pressure, no kids, time off work and good availability of movers, I'd go for a Monday. It's probably not that popular, you have a weekend beforehand with a deadline to get sorted in the old house and a week ahead followed by a weekend to get the new place sorted.
Now, if money was no problem at all, I'd stay in one of my multiple investment properties (Ha! ) while getting the new place painted, decorated and furnished so that when I was ready to move in, I could simply do it at my leisure and just turn the key. I would also spend whatever it cost to have someone else do all the packing , moving and unpacking.
mf