It’s common knowledge in the Demon Slayer world that demons turn to dust when exposed to sunlight. Yet, Nezuko is able to survive particular rays of the sun on the day she’s transformed. How did Nezuko survive the sun in episode 1?
Nezuko was able to survive the sun in episode 1 because of the cloudy weather. Giyu points out that Nezuko should be fine under the clouds, but Tanjiro has to keep her away from direct sunlight.
The siblings were lucky that Giyu was able to give them advice on a cloudy day. If Tanjiro were to carry his sister into town with different weather, she would have disintegrated into dust.
The following day, Tanjiro creates a makeshift box for Nezuko to shield herself from the sun’s rays. Nezuko is able to survive without eating by sleeping, so the box acts as a method to carry her during these time periods.
Giyu explains how Nezuko is able to survive the sun
In chapter #1, Nezuko has become a demon. Her brother, Tanjiro, unsuccessfully attempts to carry her body into a local town for medical attention. She regretfully attacks Tanjiro, crying tears before the Water Hashira, Giyu, is able to break them up.
After evaluating that there’s “something special” about Nezuko and Tanjiro, Giyu decides to leave the siblings in peace. He instructs Tanjiro to meet an old man named Sakonji Urokodaki at the foot of Mount Sagiri. He must tell the old man that Giyu Tomioka sent him.
“Your sister should be fine because the sun isn’t out today,” Giyu says. “But don’t let her into direct sunlight.”
So that’s the answer. Nezuko was able to survive the sun in Episode 1 because it was cloudy outside. It appears as though demons only burn in direct sunlight. Giyu alerts Tanjiro that Nezuko should be fine if it’s overcast, but he needs to keep her out of direct sunlight as soon as possible.