Do you mean when to trade it?
a. Ideally, you would want to track/trade it during the Frankfurt open. I mean 7-8AM London time. You can calculate it based on where you are based in.
Mark the Asian range, and then look for a break to one side. That is the lowest risk entry.
b. Or you can trade the retest.
For example, attached is the London session range for today. Not always, but sometimes you will get a retest of the session high or low. If you did not enter during London start of Frankfurt open, you can trade the retest.
But, very late in the session, avoid trading the retests. For example, NY is starting in 5 minutes. It is possible for NY to spike the previous session high lows.