Day vs. Night
As was enumerated above, there are two aspects of burning the fats: 1) as an active service of G-d, and 2) to prevent the prohibition that anything be left over.
This can be appreciated through an additional explanation as to the meaning of day and night.
Day and night are not only expressive of a person’s actions (G-dly activities vs. personal ones), but represent as well, the different situations that a person can find himself in.
Daytime: This is indicative of when the person feels inspired in his service of G-d. He consciously senses that the Torah is from G-d and that the physicality as well, has a G-dly purpose. He is not involved in the physical for himself, but instead feels G-dliness in all that he does.
Night: This is expressive of when the person does not feel G-dliness. When a person is in a situation of spiritual darkness, he experiences a constant internal struggle toward serving G-d. This conflict is not only relegated to the physical domain, in his efforts to ensure that all these activities are done for the sake of Heaven, and not for his own pleasure, but his challenge is (understandably) also in regards to his fulfillment of Torah and mitzvos.
When a person is experiencing spiritual darkness and less spiritual inspiration, he must exert tremendous energy to ensure that he is learning Torah and doing mitzvos, not for his own pleasure, but because it is G-d’s will.
While this effort of burning his personal pleasure applies both during the “day” and the “night,” the manner in which they are each accomplished is entirely different.
During the “day,” the times of his spiritual high, his entire pleasure transforms into the G-dly. When a person feels G-dliness in a revealed way, he is able to affect within himself that everything he does is infused with G-dliness.
However, during the “night,” although he cannot affect within himself to feel G-dliness in all that he does, he must at least sanctify himself to the extent that he does not do Torah, mitzvos and his own pursuits for his own gratification, but that he does them for the sake of Heaven.
This is the difference between burning the fats as an active service of G-d (the first aspect of the mitzvah), which is performed during the daytime, and the preventative aspect of burning the fats (the second aspect), which is done during the nighttime.
These two aspects parallel one’s two spiritual states as follows: when he is spiritually inspired, he is actively able to transform his pleasure into a G-dly one. When he is not, he must at least ensure that there is no negativity in his service of G-d, and that he does not perform them for his personal pleasure.
