Given the HTML:

To identify the date June 3, 2023 the xpath which you have used:
//*[contains(@class,'flatpickr-day nextMonthDay') and @aria-label='June 3, 2023']
have certain issues as follows:
- Today being June 2, 2023, the day June 3, 2023 won't contain the class nextMonthDay.
- Using contains clause it's better to pass a single attribute value.
Solution
To locate the date June 3, 2023 you can use either of the following locator strategies:
Using CSS_SELECTOR:
span.flatpickr-day[aria-label^='June 3']
Using CSS_SELECTOR:
span.flatpickr-day[aria-label*='June 3']
Using XPATH:
//span[@class='flatpickr-day ' and starts-with(@aria-label, 'June 3')]
Using XPATH:
//span[@class='flatpickr-day ' and contains(@aria-label, 'June 3')]