How can I find and click elements in my calendar?

Now try this in the browser console:

document.querySelector("div#schedulerSch .k-scheduler-content>table td:nth-child(5) > span").click()

When you have proved that works, then you can place it in your Test Case. Your code above uses j as a cell index so something like this:

for (int j = 0; j < Cols.size(); j++) {
  if(...) {
    String js = '''
    document.querySelector("div#schedulerSch .k-scheduler-content>table td:nth-child(${j}) > span").click()
    '''
    WebUI.executeJavaScript(js, null)
    ...

For background on using CSS and JS in Groovy/TestCases: