light of stars, shine
晢 centers on a clear, gentle light, especially the light of stars. It describes something that shines with a soft, distinct brightness.
晢 combines 折 (break, bend) with 日 (sun). The exact historical development is uncertain, but it likely uses 折 phonetically to express a clear, distinct light, with 日 providing the meaning of light.
The top 折 looks like a hand 扌 holding an axe 斤, breaking something, while the bottom 日 is the sun. Imagine breaking open the night sky to reveal the shining light of stars.
For セイ, picture a clear night sky where you say "say" to the stars and they shine back: say -> セイ.