A perpetual fire shall be kept burning on the altar, not go out.
Tikkunei Zohar 74a:2
One older person stood up from behind a wall, and said: Rebbe my teacher, The Holy Light, come and light candles, for that is a Mitzva, on that it is said: An everlasting fire shall be kept burning on the altar, it should not be extinguished. And, on that it is also said: To light the eternal flame. This is surely the light of the divine, the light that shines within the soul of every person. Come, light it with her.
If, however, the sacrifice he offers is a votive or freewill offering, it shall be eaten on the day that he offers his sacrifice, and what is left of it shall be eaten on the morrow. What is then left of the flesh of the sacrifice shall be consumed in fire on the third day. If any of the flesh of his sacrifice of well-being is eaten on the third day, it shall not be acceptable; it shall not count for him who offered it. It is an offensive thing, and the person who eats of it shall bear his guilt.
EATEN ON THE THIRD DAY: Torah is speaking of one who has the intention while sacrificing to eat it on the third day. One might think that what the text really means is, that if one has eaten of it on the third day it becomes disqualified retrospectively! Torah, however, states, “As for him that offers it, there shall be no מחשבה to him” implying that at the time when it is offered it can become disqualified. And the following is its meaning: At the time when it is offered, this shall not enter the mind of any priest performing a rite with it, to eat of it on the third day, and if he does harbor such a thought, it becomes piggul, an abominable thing.
piggul, offensive as in, “broth of abominable things is in their vessels” (Isaiah 65:4). … Some people have wondered how the sacrifice can be of no account once it has been “willingly accepted”. Clearly, it is the intention of the sacrificer that determines the acceptance of the sacrifice. Someone who expected to eat from the peace-offering on the third day never intended to bring a valid peace-offering …
Sifra Tzav 8:1
R. Eliezer said: Incline your ear to hear: Torah is speaking not of actual eating, but of thinking, “If one thinks to eat of his sacrifice on the third day, it shall not be accepted.”
Rabbi Adin Steinsaltz
From a halakhic standpoint, this verse is very important. Major portions of tractates Zevahim and Menahot deal with laws that derive from this passage, and there are several tractates in the Talmud that one cannot study without quickly encountering one of these laws, namely, piggul. … the disqualification of piggul relates to thought and is entirely a matter of intention. To turn a korban into piggul, one need not perform any action; the thought alone suffices.
The act of giving is fundamental to a korban … At the root of the matter, the obligation to bring a korban requires acknowledgement and understanding of the element of sacrifice that it entails. … A sacrificial act is fundamental to the process of bringing a korban, and this kind of act necessarily involves tension.
A korban is not just a matter of giving; it has an additional aspect: “A fire shall be kept continuously burning upon the Altar (Leviticus 6:6)” … This theme of “a fire shall be kept continuously burning upon the Altar” pertains to the essence of the korbanot; and just as it pertains to physical offerings, so, too, does it apply to spiritual offerings.
A human being is in a constant state of burning. There is no extinguishing this burning; it has no end, and it continues day after day and night after night. … To be sure, a person cannot live constantly in an elevated state of burning enthusiasm, and cannot tolerate a constant stream of momentous life events. Nevertheless, there is one aspect of our lives that must retain that constant spiritual high — the continuous fire.
Rabbi Yehudah Leib
” … a fire must always burn on the altar; it may not go out” (Lev 6:6). This is the purpose of human worship. … This love comes to us as a gift of divine grace. Something of this light should remain imprinted on the heart throughout day and night; “it may not go out.” When this is the case, whatever thoughts and doubts that arise upon the heart will be burned up by the inner flame of this imprint.
In fact the fire that “must always burn” is the fear of God, but the wood that “he shall burn upon it each morning” is love, the “thread of grace” that we are taught is drawn forth each day. [Hasidic prayer books begin the morning worship with this phrase.]
In the soul of every Jew there lies a hidden point that is aflame with [love of] God, a fire that cannot be put out. … there needs to burn in [the soul] a fiery longing to worship the Creator, and this longing has to be renewed each day. … Everyone who worships God may be called a priest, and this arousal of love in Israel’s hearts is the Service of the Heart, that which takes the place of sacrificial offerings. When this fiery love is present, any distracting thought that enters the heart is consumed.
So, too, the struggle in the heart of one who serves goes on “all night until the morning.”