I booked an avios flight PHL-LHR in CW for 12 Jun on the web site last night (early 24 Jun UK time), and it gave me off peak.

If they follow the same pattern as for 2015, this should be the last weekend in June giving off peak rates. For the rest of the month (and until early September) only Tuesdays and Wednesdays will be off peak. We'll see....... It complicates one's planning that the peak/off peak calendar is not pre-published for dates beyond 355 days.

On a related topic, why on earth is the YQ slightly lower for two one-way flights than for a return? For example,
PHL-LHR $441.80 + LHR-PHL $562.53 = $1,004.33.
But a return PHL-LHR-PHL is $1,189.20 ????