Recapturing the feeling of unity

I am not sure whether this is a general question or a medical one.  Several months ago I was outside in my beautiful backyard just standing around enjoying the plants and watching the birds; doing nothing out of the ordinary. In that moment, I experienced what I can only describe as pure joy.  It really has no name to me because I have never experienced these sensations before.  The air was clearer; the light was different and I felt intensely happy and hopeful.  It struck and then it went away.  I have thought about it often since and want to be able to repeat the experience but I do not know how.  Do you know what it was?  Can you help?  I look forward to hearing from you.