Most nights find Susan Tammany working at the David H. Koch Theater — dressed in black, wearing a name tag, handing out programs and helping balletomanes to their seats. But she won’t be working as an usher on Thursday night at City Ballet’s spring gala.

“I said to her, ‘Opening night, you take off — because you’ve got to take a bow!’ ” said Peter Martins, the company’s ballet master in chief.

That is because Ms. Tammany will be playing a very different role that night: designer of the sets and costumes for City Ballet’s new production of August Bournonville’s 1836 ballet “La Sylphide,” staged by Mr. Martins, which is having its premiere at the gala.

For Ms. Tammany, an artist by day who moonlights as an usher, the job was a reunion of sorts. She designed “La Sylphide” when Mr. Martins first staged it 30 years ago for the Pennsylvania Ballet, in a production that was later broadcast on public television. But she said she was stunned when Mr. Martins approached her about doing it again at City Ballet.