Gala premiere, then quiet 30th for Kate Middleton (AP)

The Duchess of Cambridge talks to a guest after being presented with her bouquet as she arrives for the UK Premiere of 'War Horse' in aid of The Foundation of Prince William and Prince Harry, at a central London cinema, London, Sunday, Jan. 8, 2012. (AP Photo/Ian Gavan, Pool)AP – After attending a gala film premiere in a full-length lacy evening gown, Kate Middleton stepped out of the public gaze Monday to celebrate her milestone 30th birthday in private.