My "rule of thumb" for that has been:
If by Wednesday evening/Thursday morning the high is lower than the previous week's high, then sell on Wednesday evening / Thursday morning and buy those back on Sunday afternoon -- before Asia wakes up.
But don't whine if, like what happened on Easter weekend, getting back in on Sunday afternoon costs you more than you made by taking advantage of previous weekend dips.
[Caveat: I'm not your financial advisor. If your luck at timing the market is as good as mine, then listening to me will likely cause you to lose some or all of your money. ]