Exodus 27:21
In the tabernacle of the congregation without the vail, which is before the testimony, Aaron and his sons shall order it from evening to morning before the LORD: it shall be a statute for ever unto their generations on the behalf of the children of Israel.
Aaron and his sons were given charge of the lamp in the tabernacle of the congregation before the Lord, to order and tend it through every night from evening until morning. Its burning was to be continual and unbroken, never permitted to grow dim or to go out. That light was not furnished to illuminate the heavenly sanctuary, but stood for God's church giving light to the world, with Israel appointed to uphold and represent that great light so that it would never become extinct.
When the temple services were interrupted under Ahaz, the candlesticks were left unlit, in direct violation of this requirement to keep them burning.