Whenever I have the opportunity to lead worship, I greet the people who are gathered with these words; "This is the day that the Lord has made, let us rejoice and be glad in it." (Psalm 118:24) I begin with these words because (a) I do believe that each day is a gift and (b) that the appropriate response to a gift is joy and (c) I am eager to set a tone of gratitude and joy for the service as it begins. But there is more to it then that. Many days are joyless. Frederick Buechner has written the following which begins to get at the depth of the meaning of these words better then I can.
It is a moment of light surrounded on all sides by darkness and oblivion. In the entire history of the universe, let alone in your own history, there has never been another just like it and there will never be another just like it again. It is the point to which all your yesterdays have been leading since the hour of your birth. It is the point from which all your tomorrows will proceed until the hour of your death. If you were aware of how precious it is, you could hardly live through it. Unless you are aware of how precious it is, you can hardly be said to be living at all.
"This is the day which the Lord has made," says Psalm 118. "Let us rejoice and be glad in it" (v. 24). Or weep and be sad in it for that matter. The point is to see it for what it is, because it will be gone before you know it. If you waste it, it is your life that you're wasting. If you look the other way, it may be the moment you've been waiting for always that you're missing.
All other days have either disappeared into darkness and oblivion or not yet emerged from it. Today is the only day there is.