When did G‑d create light?

Question:

If you look at Genesis, you will see that G‑d created light on Day One. However, Scripture also says that G‑d created the sun, moon, stars, etc. on Day Four. So what was this light then that was created on Day One?

Answer:

The Talmud1 cites two views. One is that the light created on Day One was extremely brilliant, but G‑d foresaw that the world was undeserving of it, so He concealed this light for the righteous in the hereafter. According to the second opinion, however, the light created on Day One was in fact the light from the luminaries -- which were then "hung in the sky" on Day Four.

Rabbi Eliezer Danzinger for Chabad.org