I design a form in which the user must select his birthday. only day and monthnot the year.
Since this is only the day and the month, the standard procedures for date picker programs do not necessarily apply, though some do.
Nielsen and Norman recommend splitting the fields and, if possible, specifying the month. UX movement recommends the use of plain text fields. This excludes the possibility of spelling the month. There is also the question of whether the focus should automatically be moved to the next field – Lukas Mathis recommends talking against it, but without this, an extra click is required if two fields are used.
I should also mention that the UI should only consist of standard elements. I know there are big custom pickers, but this project does not have the budget to build one for such a small feature.