That’s how he knew…

After packing up Rainbow Dash’s little secret knapsack and putting on their dark disguising robes, they set off along the edge of the Evergreen Forest and decided to head East. Dash was always in the lead or next to the Alicorn, and occasionally they had little fun races where Rainbow could check on Twilight’s progress in flight and give her additional little pointers and tips on how to fly “about 20% more awesome!”

Twilight grinned as she was lectured playfully like this and steadily picked up speed throughout the day, Rainbow Dash laughing as they soared between the clouds, figuratively kicking up dust.

By nightfall, they had come across a small travelers cabin and settled down on the small cliff it was located at, Twilight quickly making a magical outside fire for them. They sat perched on the edge of the cliffside and eating strawberry ice-cream.
Twilight laughed, magically pulling away from her, as Dash stole a bite of her cone. She smirked at the Pegasus, leaning in to kiss the smudge of chocolate off those sweet lips. The latter made a pleased noise in the back of her throat as their mouths remained connected for a delightfully long time before they pulled apart for air.
"Thanks for today," Twilight said, contentedly.
Rainbow scooped up a spoonful using her wing as a hand and offering it to her as she replied, "Anytime."

Before Twilight could bite into it, Dash suddenly straightened, catching her off guard. She looked out above the trees. Tilting her head slightly, listening carefully. She then stood up and flew slowly away from her spot.

"What is it?" Twilight asked.

"Hey Twi, get up. We've got to get inside."
Twilight quickly got up now as well. She followed her, storing her ice-cream in a Magic portal and feeling a bit confused… until she was suddenly engulfed in a terrific downpour. Then things became abundantly clear.
"Really? Rain?”

Dash retreated into the small tourist cabin they had found, the door not being to creaky as she pushed it back open. This was why they chose to stay here on this particular cliff, and the beds looked comfy enough for the night. The rain came down in torrents behind them, and Twilight was only too glad to be back in the cabin now, quickly moving over to the fireplace and activating the Fire Rune located on the wooden table next to it, causing the mystical flames to immediately appear and start spreading their comforting heat throughout the place.

When she turned back around, she saw Rainbow discarding her soaked coat. The way the rain really soaked her cyan fur to form-hug her athletic figure even more was really distracting, in a welcome way. Twilight let her eyes sweep over her marefriend appreciatively for a moment before busying herself with her own jacket. She easily teleported her wet outfit off her body and floated it to the fireplace, hanging it up on a nearby hook that was installed specifically for this purpose to slowly dry it out along with Dash’s jacket as well.
She soon felt eyes on her now. Looking back, Twilight found the Pegasus looking over her in a similar fashion, only doing it longer and with a smirk.

She chuckled. It was nice to know it wasn't only her. Drying her hooves on the front of the fire, Twilight giggled a bit as she felt Dash come up next to her and start doing the same.

“Y'know, you could probably just warm us right up with a single, I dunno, Drying Spell or something?”
The cyan Pegasus said with a chuckle.

“What? Is the great Rainbow Dash wanting to use Magic to solve her problem of being wet?”
Twilight said with mock surprise and faux shock, and this earned a laugh out of them both.
After another minute, Rainbow then pulled away and retrieved her main piece of property from her sack. It was a small magical mixtape, containing a number of old songs and musical pieces from previous decades before she was born.
Twilight did not fail to notice bow Dash’s entire demeanor had quickly changed at this point, and she was now quieter and more solemn. The Pegasus had a deeply emotional look in her eyes, and she sat next to her marefriend again as she flipped the switch and a song Twilight had never heard before started to softly play.

“My mom created this collection when she was around my age, before she met my dad, you know.”

Twilight smiled, and gently reached out a hoof and rubbed Rainbow’s back.
“Thank you for letting me hear it, Dash. It means the world to me for you to trust me this much.”

“It’s not just about trust, Twilight.”
The Pegasus answered, and the quiet tone of her voice convinced Twilight to simply stay silent and listen to the songs with her. This clearly was very important for Rainbow and a HUGE step forwards in their relationship, and the Pegasus was in a very different mood now than she tended to be when they were together. This was very personal for her.

The mix-tape crackled as the final track came to a close after several long minutes. It made a small magical tinkling sound, then it ejected and Twilight caught the tape and put it down gently on the floor telekinetically. There was now nothing left, but the rush of the rainstorm outside to fill their ears, and she saw that Rainbow was still sitting silently, not moving or speaking.
Twilight looked from Rainbow toward the tape deck. “I should probably give that back, now. Sorry for now doing it earlier.”
She whispered.

Rainbow shook her head and reached for the mix-tape without a word. She picked it up, put the tape back into the device, and now sat down a few hoof-steps from Twilight, holding it in one hoof and running her other one across the writing, along the sides of it. It was her mom's writing.
“It was on the night of their third date that she gave it to him, and he finally knew.”
She said, her voice uncharacteristically soft again.

"Knew what?" Twilight asked.

Rainbow’s eyes flicked up to hers. "That she loved him." She replied. “Would you do something for me? Please?”
She asked, and her face now looked suddenly more vulnerable than Twilight had ever seen it.

"Yes. Anything.” The Alicorn confessed without missing a beat. Whatever it was, she just felt that Rainbow needed her to say yes to this question, and she meant what she said when she answered.
Rainbow calmly took a few steps closer to her, reached across the space between them… and slipped the mix-tape into Twilight’s own hooves before sitting down again.
"I want you to keep this for me." Rainbow said, her voice barely above a whisper.

Twilight looked down at the mixtape in her hooves and then back up, her wide eyes locking with Rainbow’s surprisingly calm ones.
"Rainbow—" she began, her voice cracking, her heart hammering in her chest.
"— I want you to always have a part of me with you, Twi. To know you're not going to lose me. That I’ll always be with you, even when I’m not physically present the way I am right now.”

"Rainbow..." Twilight rasped. "You can't just say things like that and-and not expect me to come over there."
Rainbow Dash smiled at her and blinked in a flirtatious manner, "Well c'mere to me then."

Twilight gently put the mixtape down and quickly got up and crossed the short distance between them. Taking Rainbow’s face in her hooves, she kissed urgently and deeply, pouring everything she had into it. And as Dash pulled her onto the floor so that Twilight could straddle her, the Alicorn knew she would never think about the mixtape, this location, or even the rain itself in the same way ever again.

Whenever she would think of them in all the years yet to be, she would now only be able to remember this one special night and this one special moment. The first time that she truly knew, without a shadow of doubt…

That Rainbow Dash loved her.