The Light of the first day refers to Hashem's intent and goal in Creation, and is therefore God's Will. Will always precedes, and acts as the power of, Wisdom.
God's purpose is that the world be transformed to Light, i.e. His very Self should find a dwelling in the lowest world.
The Light of the first day was hidden away, and a ray of it was channeled into the creation of every detail of all that was created. In this mode, this small vector of a ray is more associated with Kindness. The act(s) of Creation is indeed a pure act of Kindness, as what is Kindness if not giving. In this mode, the light is hidden, and it is the mission of the Jewish people to expose it through Torah and Mitzvot.
The Torah is where God hid the Light, and its Wisdom is indeed associated with the Light as God's desire is expressed as a plan; a blueprint, and hence the association with Light and Wisdom (Sotah 12a). The Light is also associated with Goodness, which is what every phrase "it was good" is really referring to - the Light that is in all the details of creation, that will one day be revealed.
A practical lesson, that also helps us understand this deep idea, is how we also go about fulfilling our desires. At first, we transform the desire into a plan, and then we hide away the desire somewhere in the background, while we execute the plan. The desire is what empowers the entire process, but during that process it is not focussed on, but when the plan is accomplished, the desire is able to be fully revealed once again, in enjoying the fruits of the labour.
See this Sicha for more information, explanation, and plenty of sources.