G‑d told Moses that once a sacrificial animal has been slaughtered, the parts of it that are to be burned should preferably be placed on top of the Altar during the day. However, if this is not possible, they may still be placed on the Altar-fire any time throughout the following night.
The Eternal, Inner Flame
וְאֵשׁ הַמִּזְבֵּחַ תּוּקַד בּוֹ: (ויקרא ו:ב)
[G‑d told Moses that a sacrifice is still valid if it is placed on the Altar at night,] for the fire of the Altar must burn on it [throughout the night]. Leviticus 6:2

The Altar fire that was kept burning throughout the night was kindled during the day.

The Altar alludes to the Jewish heart. Even when we find ourselves in situations of spiritual darkness, we must keep the Divine fire of enthusiasm for G‑d, His Torah, and His commandments, always burning in our hearts.1