For things like this, prefer to use JavaScript:
els = driver.execute_script("""
return Array.prototype.slice.call(document.getElementsByClassName("fc-day-content"))
.filter(function (x) { return x.textContent === "15"; });
""")
assert len(els) == 1
el = els[0]
What this does is get all elements that have the class fc-day-content as the class. (By the way, your question uses fc-day-content and fc-day-number. Unclear which one you're really looking for but it does not matter in the grand scheme of things.) The call to Array.prototype.slice creates an array from the return value of getElementsByClassName because this method returns an HTMLCollection and thus filter is not available on it. Once we have the array, run a filter to narrow it to the elements that have 15 for text. This array of elements is returned by the JavaScript code. The assert is to make sure we don't unwittingly get more than one element, which would be a surprise. And then the element is extracted from the list.
(If you care about IE compatibility, textContent is not available before IE 9. If you need support for IE 8 or earlier, you might be able to get by with innerText or innerHTML or you could check that the element holds a single text node with value 15.)
I prefer not to do it like TehTris does (find_elements_by_class_name plus a Python loop to find the one with the text) because that method takes 1 round-trip between Selenium client and Selenium server to get all the elements of class fc-day-content plus 1 round-trip per element that was found. So if you have 15 elements on your page with the class fc-day-content, that's 16 round-trips. If you run a test through Browser Stack or Sauce Labs, that's going to slow things down considerably.
And I prefer to avoid an XPath expression with @class='fc-day-content' because as soon as you add a new class to your element, this expression breaks. Maybe the element you care about has just one CSS class now but applications change. You could use XPath's contains() function but then you run into other complications, and once you take care of everything, it becomes a bit unwieldy. See this answer for how to use it robustly.