This could be just before the Grand Galloping Gala or more likely afterwards. In my headcanon, Flash was beginning to become so discontented with the Order he never went out with pegasi fillies in his school. He would have had infatuations, but only with unicorn mares.

Finally! Something to waste my mind on!
I'll take a shot into the blue: It's gonna take it's time. It shouldn't be something like 'Friendship between Male & Female' ---> BOOM! ---> 'Luv!'. No, instead it will slowly develop over several chapters - just like before. And it will be good! There won't be a certain point where it turns, but a few little peeks into something deeper as well as a bunch of precious moments until we realize it's too late!
*Dramatic Pause*
... or Discord spills the beans right onto the floor since I'm pretty sure he read the whole mind of Flash or something like that...
However, the GGG should be one of those precious moments like the cliff~

Ah, sound like I made your day. I imagine Flash shaking Blue Blood off for a few minutes for a quiet talk with Twilight in the Gardens or a side-room.

4459108 Could (providing Twi. wrestles Flash out of Blue Blood's hoofs) Flash and Twi. use Twi.'s monthly visits to Ponyville as undercover romantic dates? For example: astronomy observation dates, flying lessons or sky-dances, and normal dancing and flirting at Pinkie Pie's partys?

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Shame that nobody else want's to waste their minds on that.

Huh, wouldn't it be a bit suspicious if they both are missing? At the same time? I'm preeeeeety sure the Captain will keep an eye on her all the time by now, since he's interessted in her by himself. He wouldn't tolerate any action around her which doesn't fit him, especially not if Flash is involved. I guess without Celestias, Lunas or even Discords involvement nothing is going to happen all too soon.

A colt can imagine can he not? And if the two of them were found out; what would Flash's punishment be? A dishonourable discharge and immediate expulsion or a locking in the dungeons? Bad came back to me on one of my story ideas. She told me that Flash would be sacked on the spot before he could stammer an apology. Celestia would protect her old student from the dishonour.