The red dragon has been flying from the roof of No 10. Inside, invited guests - including half the cabinet - enjoyed Welsh produce from Welsh cakes to whisky and wine.
The prime minister, who has given up crisps for Lent, chose not to sample all the food and drink - "I've had to be very good and not participate in everything that was brought through" - but suggested it was so impressive quite a few No 10 staff "have suddenly discovered their Welsh heritage".
There was, she said, much to celebrate in "this proud country - a very special part of our United Kingdom", including the hills. "And around Easter I hope to be back in Wales walking up some of those hills once again."
Her message was a familiar one - she is looking for a Brexit deal that works for the whole of the UK. She said: "I am determined that we will emerge from this great period of change stronger, fairer, more united and more-outward looking than ever before. And that will be the best possible outcome for Wales - and for the whole of our United Kingdom."
'One of the best'
On policies, she highlighted the UK government role in part-funding the Cardiff Capital Region City Deal. "And why we remain committed to negotiating a City Deal for the Swansea Bay City Region too." (Chancellor Philip Hammond suggested to MPs yesterday that there may be an announcement in his Budget on March 8).
Guests, who included Falklands War veteran Simon Weston, cyclist Nicole Cooke and chef Tom Simmons, were entertained by Côr Heol y March from Cowbridge.
Earlier, during prime minister's questions, Theresa May said Wales was "one of the best places in the UK to live, to work, and to trade with" as she wished MPs in Welsh "Dydd Gwyl Dewi Hapus".
She added: ""In the forthcoming negotiations we are committed to getting a deal that works for all parts of the UK, including Wales. The best way to achieve that is for the UK government and the devolved administrations to continue to work together."
But with the threat to jobs at the Ford plant in Bridgend also featuring during question time, the prime minister's warm words and celebrations were rather overshadowed by the economic challenges facing "this very special part of our United Kingdom".