Kate is expected to mark the occasion privately with the Duke of Cambridge and her children Prince George and Princess Charlotte, ahead of her first public engagements of the new year.
There were no clues about how she would spend the day when she appeared with the Royal Family at church on Sunday in Sandringham, where the Queen was in attendance for the first time since a heavy cold kept her away from services over the festive period.
Kate, wearing a grey furry hat and a forest green coat and black heels, walked the short distance to the church with William. But there was no sign of George or Charlotte.
The rest of the Middleton family - Kate’s sister Pippa, mother Carole, father Michael and brother James - also attended the service.
Royal watchers are waiting to see whether 2017 will be the year the Cambridges expand their brood.
Kate, who is one of three herself, is thought to be keen to have another child.
George, who turns four in July, is due to start school in September.
Although no official announcement has been made, he is expected to attend the same one as his father William - the £6,865-a-term Wetherby Pre-Preparatory School in west London.
It would mean a move back to Kensington Palace for the Cambridges from their country retreat, Anmer Hall in Norfolk.