I appear once in summer and twice in autumn, but never in winter. What am I?

Answer: The letter 'u'

More What am I ? with answers

More What am I ?

I ask no questions but receive a lot of answers. What am I?
I beam, I shine, I sparkle white. I’ll brighten the day with a single light. I’ll charm and enchant all. I’ll bring the best in you all. What am I?
I begin and have no end. Eventually I will be the ending of all that has begun. What am I?
I begin your sentences. What am I?
I belong to everyone. Sometimes make you happy, sometimes make you sad. I will never end until the day you do. What am I?
I belong to you but others use me more often than you do. What am I?
I build up castles. I tear down mountains. I make some men blind, I help others to see. What am I?
I can alter the actions of a king and leave puzzled the greatest of philosophers. What am I?
I can be cracked, I can be made. I can be told, I can be played. What am I?
I can be cracked, made, told and played. What am I?
I can be cracked, made, told, and played. What am I?
I can be deep, I can be powerful, I can be complex, blind, difficult, and profound at the same time. What am I?