title: R function to select specific week days
body: <p>I am working on a project and would be happy about your help.</p>
  <p>I am working with stocks and the effect of weekdays on performance, is there a way to take all the values (for instance the S&amp;P 500) of a data frame (df) from a specific weekday (e.g. Tuesday) and enter these values in a different data frame (df2) in a new column?</p>
  <p>Thank you very much,</p>
  <p>Ferdinand</p>