LONDON (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.

She turned heads — and dominated Britain’s Monday morning front pages — by attending the London opening of Steven Spielberg’s “War Horse” wearing a striking design by Alice Temperley. Her husband, Prince William, used an umbrella to keep her glossy brown hair drizzle-free.

It was another star turn for Middleton, who has appeared ever more comfortable in public since joining the royal family in April.

Palace officials said the Duchess of Cambridge’s birthday celebration will be a “low-key and private” affair. The choice of a muted event seems appropriate: After all, the duchess is not just starting the fourth decade of her life — she is also embarking on her career as a future queen, charged with renovating a monarchy that has become somewhat creaky with age.

“She’s done very well, playing a very good supporting role to Prince William and complementing him as part of a team,” said Robert Jobson, author of “William and Kate: The Love Story.” ”We’ve not seen much of her, but when she’s in public she has performed with aplomb.”

The past year has seen momentous changes for Kate, with a fairytale wedding to Prince William, travels around the world and glamorous appearances that marked her transition from commoner to the world’s most talked-about princess and style icon. She has won rave reviews for her sparkle, her elegant sense of style, and her common touch with the public.

But now that the excitement around her grand wedding has subsided, she is expected to quietly settle down into her royal duties — and for much of this year, that means helping Queen Elizabeth II celebrate her Diamond Jubilee, without upstaging the monarch in her moment of glory, and also playing a supporting role at the London 2012 Olympic Games.

Until recently, palace officials have been quiet about what exactly her role will be, keen to protect her transition into a fully fledged royal from the glare of the media limelight. The newlyweds, who live in a remote corner of north Wales, were said to favor a “quieter life” after the massive global media coverage of their April wedding.

The former Middleton ventured out on her first solo engagement in October — a private charity dinner for about 30 guests — but is yet to make a public speech on her own.

That is set to change. This week, royal officials finally gave the first insight into her new role when they announced which charities she’s supporting, including the Scouts and a group that helps drug addicts. More organizations will follow later, officials said.

She has already taken on an unofficial role as a champion of the British fashion industry, wearing the creations of British designers at home and abroad, showcasing their work in the best possible light.

On top of such duties, the duchess also has a busy year ahead with the queen’s jubilee, a major celebration of the monarch’s 60 years on the throne that will see numerous celebrations throughout Britain and the Commonwealth, peaking in a major celebration in London in early June.

The royals are expected to play a supporting role by traveling overseas to represent the queen. William and Kate are due to journey to Malaysia, Singapore, the Solomon Islands and the tiny Pacific island of Tuvalu as part of the festivities.

Inevitably, much of the speculation about the duchess’ future has centered on royal lineage — Is she going to start a family soon? That, however, is anyone’s guess, and despite incessant tabloid baby chatter, there doesn’t yet appear to be great pressure on the newlyweds to produce an heir.

“It’s early days yet, and they’re both getting used to their roles,” said Jobson, adding that the presence of other young members of the royal household, such as Prince Harry, helps to ease the pressure on William and Kate.

Many of the more senior women in the royal family were mothers in their 20s, but times and expectations have changed — both inside and outside the royal household.