Turn again, and tell Hezekiah the captain of my people, Thus saith the Lord, the God of David thy father, I have heard thy prayer, I have seen thy tears: behold, I will heal thee: on the third day thou shalt go up unto the house of the Lord. (2 Kings 20:5)
That is, on the day after the morrow Hezekiah would be well enough to make his way to the Temple to give praise to God. It is implied that Hezekiah’s first act upon recovering would be to return thanks to God in His Temple.
I will heal thee.
God could have healed Hezekiah instantaneously, but He did not choose to do so.