We are all waiting for that one person to make a huge J/F $ booking to a land far-far-away and behold, QF drops a DSC
I remember in 2018/19, even early 2020 I cancelled and re-booked at least 6 Y international trips because QF decided to drop a DSC, right after I had paid for the 6 tickets. Luckily I had booked all in Flex Y, so it was easy to change.
On the topic that OP had queried - if my memory serves right, QF usually does a DSC in Feb/Mar and another in Sep/Oct ... some years we have had 3 DSC, most years 2 DSC promos. VA normally follows suite in Apr with a DSC. The Feb/Mar DSC usually is for everyone (meaning, not targeted) and the booking window is for 7 days from the launch of the promo, travel window up to the end of Feb next year. Following this suite QF had a promo Mar/Apr this year for QF flights ex-AU.
Then in 2018/19, QF also had this thing were one could "trigger" a DSC based on the searches & taking a booking up to the point of payment and then stop. The application then creates a temporary PNR which is on hold for 24 hours. AFter the 24 hour window, a DSC is "triggered" for the route searched or the source/destination.. I was able to successfully "trigger" DSC on a few occasions this way.