We celebrate the source of our great joy, coming out of slavery, getting through that dessert experience & entering into the promise land. God wants us in an area of great joy, he doesn't want us being at the back foot, a place where we have no peace, oppression, suppression, in a place where we have been challenged with our identity. We'd say we're not going through a good patch in our lives now, the doctors reports, things happening in our businesses, in our jobs, things we're experiencing in our bodies right now, it's not a happy time for us, it's ok because joy on the contrary has nothing to do with happiness. The opposite of happy is sad. Sorrows may last for the night but joy comes in the morning... There's no substitute for joy,  all the enemy wants is us to turn away from joy because in the absence of our laughter, of our tears, even in happiness we will not experience joy, but when we don't have happiness, when we're in a time of sorrow the joy does not disappear because the source of our joy is important cause our God says, he'd never leave us nor foresake us, he'd be with us now till the very end of age. There's no condition, situation, medical report that can take our joy away. It may not be a happy time, we may be sad, in a time of sorrow, but our bible says, in the absence of all these things because there's a source of joy that will not be broken we will live in joy. Our source can always be challenged, what we receive is dependent on our source. There's no challenge big enough for our God. Our bible says, is there anything too difficult for our God to do... 
God cannot fail, he will not fail, he will always fulfil his word, there's nothing that can cut us off from our joy because nobody can overcome our source, there's no debt, death, grave, no illness, no challenges, nothing & nobody can take our joy because there's a flow of joy because the source has no limits. There's no limits to our God. Jesus said, "If anyone who's thirsty come unto me & let him drink & through me will flow rivers of living waters "...