Catherine, Duchess of Cambridge in Roland Mouret attends the Royal film performance of 'Mandela: Long Walk to Freedom' at Odeon Leicester Square on December 5, 2013 in London, England. (Photo by Chris Jackson - WPA Pool/Getty Images)

Katy Perry in Naeem Khan attends the 9th annual UNICEF Snowflake Ball at Cipriani Wall Street on December 3, 2013 in New York City. (Photo by Craig Barritt/WireImage)

Selena Gomez in Zara poses backstage at 106.1 KISS FMÂ's Jingle Ball 2013 American Airlines Center on December 2, 2013 in Dallas, Texas. (Photo by Gary Miller/Getty Images for Clear Channel)

Ariana Grande performs onstage during 106.1 KISS FMÂ's Jingle Ball 2013 at American Airlines Center on December 2, 2013 in Dallas, Texas. (Photo by Christopher Polk/Getty Images for Clear Channel)

Rita Ora in Vivienne Westwood attends the British Fashion Awards 2013 at London Coliseum on December 2, 2013 in London, England. (Photo by Mike Marsland/WireImage)

Lea Michele in Dolce & Gabbana attends L'Oreal Paris' Women of Worth 2013 at The Pierre Hotel on December 3, 2013 in New York City. (Photo by Paul Zimmerman/WireImage)

Sienna Miller in Burberry attends the British Fashion Awards 2013 at London Coliseum on December 2, 2013 in London, England. (Photo by Stuart C. Wilson/Getty Images)

Allison Williams in Oscar de la Renta attends the 2013 Winter Ball For Autism the at Metropolitan Museum of Art on December 2, 2013 in New York City. (Photo by Jim Spellman/WireImage)

Britney Spears in Antonio Berardi celebrates her official arrival at Planet Hollywood Resort & Casino on December 3, 2013 in Las Vegas, Nevada. (Photo by Isaac Brekken/WireImage)

Amanda Seyfried in Zac Posen attends the Cle de peau BEAUTE 'Muse' party on December 4, 2013 in Seoul, South Korea. (Photo by Chung Sung-Jun/Getty Images for Shiseido)