According to folklore, you can wish...
When blowing out your birthday candles
When Tossing a Coin into a fountain or a well
When Breaking a wishbone, and being on the receiving end of the larger half.
When you lose an eyelash, and blow it from the tip of your fingers.
When the clock strikes 11:11
When you see a falling star
Upon the first star you see at night
On a necklace clasp that has turned the wrong direction
When blowing the seeds from a dandelion in one breath
When you see a white horse
When you catch a feather
When a lady bug lands upon you
When you hold your breath in a tunnel
Am I missing any?
I'm not sure where most of these originated from, but they're the ones that I remember from my childhood. I'm not a superstitious person, but I still find some joy in wishing upon certain things...or planting the seeds of a dream.