When Nezuko was young- and she says this as if she isn't still young, isn't still a child hardened by strife and circumstance- she would watch her father dance the night away despite his fragility, watch the torches set around him flicker and dance as he twirled, once, twice, three times, his arms forming elegant and precise patterns untiringly. She'd tug her mother's sleeve and ask, "Why can't we join in too Mother? It looks so fun" and her mother would look down at her and smile, readjusting her sleeping older brother in her arms as she answered, "That's because it's a sacred dance, Nezuko; I told you this."
Nezuko would frown a bit, "But wouldn't the Fire God be even happier if all of us did it together? Then, then, the more people who did it the less people getting sick right?" She would look towards the circle of fire, excitement ignited within her so easily and then back at her mother pleadingly, but the answer was always a soft "No, Nezuko" and she'd scowl harder, put off by what was in her eyes a pointless refusal.
After all, one day she and her siblings would dance for the Fire God anyway. Why not start young?
Of course, she hadn't known it was terribly difficult to learn initially at the time. She hadn't known a lot of things.
All she knew was that she was able to stay awake for far longer than her older brother during these yearly rituals, who despite his promises and equal excitement would nod off eventually, whereas she would stay up despite her mother's coaxing to come to bed. When her mother would finally go off to sleep and hours after that their father would finally finish, he'd find Nezuko stubbornly seated just a bit away from him, gazing up with sleep deprived yet unflinching eyes. And he'd notice the fire in her eyes and the uncharacteristic solemness of his second born in that moment but the moment would be shattered with its shards forgotten right when she'd tackle him into a hug, grasping his legs with adoration and exhaustion as she'd beg him to teach her and Tanjirou so they can do it with him every year and can Mom join too?
Every time, he'd laugh and gently pry her off, taking her up onto his shoulders without her requesting such and without any complaint despite the hours of dancing. The next morning she'd wake up in her bed, having presumably fallen asleep on his head and never having had her questions answered.
Before his death, on such a New Years and on a similar sunrise, he had placed a hand on her head and finally said, "But Nezuko, you and Tanjirou already know the dances don't you? That's why I didn't feel the need to teach you both formally."
Nezuko had stared up at him, gobsmacked.
Her father had continued looking down at her patiently, "I've seen Tanjirou and you practicing on occasion, you know. Did you both want to surprise me?"
She'd puffed up her face, "No, that's silly! That's just play, Father, it's for fun! It's not at all as pretty as yours!"
He'd just smiled at her patiently and shook his head.
"That's enough. If you both ever truly need it, you'll remember. You both have keen senses and you've been staying up to watch when you were so very small", he bent down to her level and pulled her in for a hug, "so I have no doubt that you two will continue the tradition."
"Father...", she had clutched onto him- something about his statement had frightened her. Confusion turning to fear turning to indignation, she'd pounded at him lightly with her fists, "Don't talk like that! Don't... Don't talk like you're going to leave! You and me and nii-chan and our younger siblings, we'll all learn and then...!" She'd pulled away from his grasp, just enough to look at him directly when she declared with a toothy, determined grin, "You'll be the one watching us, just like how we watch you! A-And the Fire God will be so impressed that you'll get better too!"
Her father wasn't the type of man to get phased easily, she'd known that. And yet at that moment, as he brought up his hand to ruffle her head while continuing to smile at her, she'd thought that his eyes seemed to glisten a bit in the firelight.
"I look forward to that with all my heart, my dear Nezuko."
When their father passed away, Nezuko was older than she had been when she'd made this promise. And she'd realized abruptly- as she held onto Rokuto who couldn't seem to stop sobbing in her arms despite her attempts, as Tanjirou and Hanako supported their mother one on each side as she wept softly, holding both their hands, as Takeo sat in shock and Shigeru kept calling their father's name desperately- that their father's statement all that time ago had been said with such finality that it had shook her.
Her heart rate, which she'd forced herself to remain clam if only for Tanjirou and her mother's sake, the former who was holding it together for everyone, suddenly spiked.
'He knew?', she clutched Rokuto as her eyes widened, 'He knew he was going to die? Then why...'
Nezuko had gotten up. She had extracted Rokuto's clumsy, snotty grip from her kimono, handing him over to Shigeru and walked towards her now deceased father, who's kind face was covered in white cloth and stood there, barely aware of how Tanjirou was gazing at her in worry even as his own eyes glimmered.
'Ah', she'd thought, 'He passed away in his sleep, still wearing the same smile he did in life.'
The frightening expression did not leave her face even for a single moment.
'...The smile they'd never see again.'
Her heart had ached fiercely.
'You shouldn't have promised', her mind had screamed in grief, 'Didn't you know how cruel it was? Didn't you know how much we love you? Didn't you know how much I wanted us all to do it together, even though Mother always said no and you always said one day? Why did you call me your dear Nezuko? If I'd known you'd leave us like this- like this- like this...!'
Her mouth had seemed to move on its' own, her voice coming out as a hoarse whisper, as if she'd been sobbing for hours.
"Why... Did you promise...?"
She had made the mistake to blink and a tear spilled out, trickling down her face.
Something snapped- her forced composure.
Her darkened expression finally relaxed just as her knees gave out, a howl wrenched itself out of her throat and turned into broken wails. This had a mild chain effect in helping Takeo out of his shock and into tears as well, which she was distantly aware off as she heard her little brother's wails join her own.
Tanjirou had been the one to rush in and envelope her in a hug which she had returned, nails digging into him as if she intended to rip his haori off. Her sobs had been muffled just as Tanjirou's silent tears and hiccups went unnoticed by all except her, the fabric on her shoulder getting wetter and wetter by the second as he had gripped her like she was his lifeline in this particular moment.
'And in a way', Nezuko had thought grimly, 'I suppose I am'. She and Tanjirou would have to step up now that their father was gone, for their mother and their siblings' sake. They'd always done whatever they could but now they would try harder, do more, be more- so that their mother wouldn't have to feel alone, so that everyone would smile once the pain lessened. If it ever lessened.
Because at that time, it had felt like the world had ended, after all.
Nezuko hadn't known a lot of things back then, of course.
She hadn't known that the world can end for someone multiple times. She hadn't known that a hard fought for happiness can end so abruptly, stolen so easily, with a disgustingly strong scent of blood that didn't require Tanjirou's superior senses to pick up on, with a room decorated in the blood of her family, with a home that will never be warm again.
Nezuko had thought that when her father passed away, when the fire of his waning spirit finally extinguished like a blown out candle, she had understood the meaning of 'permanence' and 'all things ending eventually.' Unlike Tanjirou, she didn't spend anytime blaming herself- she knew they couldn't have done anything more than what they had.
She had thought that eventually, things would settle and they would be alright.
It is at age 12 that Nezuko realizes, as children do with many things in whatever circumstance, that she hadn't known anything at all.