The Call of the Desert is unanticipated and in most cases, unwelcomed. Which is to say that spiritual deserts are disorienting and misunderstood.
You do not choose your deserts. Your deserts choose you.
The call appears when you face unexpected, or overwhelming adverse change in the MWAIWITB; my-world-as-i-want-it-to-be.
You will ignore the call of the desert. You will resist the call. You will fear the call to leave the convenience of your man made city, but each time that you say "no" to the call, your adversity will grow darker and more mean.
Accepting the call will result in one of your most satisfying spiritual seasons of reflection and learning.
Some truths can only be learned while doing desert time.
The desert is a place where a strange love goes to work on your ego. The love is divine. It is not human. This love is so transcendent, that it can be misunderstood.
The aim of God's love is to strip you of your sense of control, redefine your self-identity, purify your motives, and tutor the mind in the ways of divine wisdom.
There are no clocks in a spiritual desert. Time feels to stand still. The length of stay is not important. The depth of inner change is the measure.  
When you leave your desert, your mind will be reorganized at a deep level. You will think differently. Therefore, your priorities will change, because new choices always flow from a renewed mind. 
When you reenter the man made city, you will have a life-giving message to share.