Nehemiah 8:10 (NASB95)
Then he said to them, “Go, eat of the fat, drink of the sweet, and send portions to him who has nothing prepared; for this day is holy to our Lord. Do not be grieved, for the joy of the LORD is your strength.”
The biblical word joy has been interpreted as inward pleasure or delight not based upon circumstance. It has the sense of being glad despite situations. Joy differs from “happiness,” which depends upon external happenings. In other words, things can occur that cause our emotions to change from happy to unhappy.
The Hebrew word rendered "strength" refers to “a place or means of safety, protection refuge, or stronghold.” The root word means “to be strong, prevail; to make firm.”
Since God does not change, the joy of the Lord is a constant source of pleasure, delight, and gladness. Therefore, just being His children is reason enough to rejoice, regardless of what we are facing.