Or shorten David’s answer with:
if 4 <= day <= 20 or 24 <= day <= 30:
suffix = "th"
else:
suffix = ["st", "nd", "rd"][day % 10 - 1]
Or shorten David’s answer with:
if 4 <= day <= 20 or 24 <= day <= 30:
suffix = "th"
else:
suffix = ["st", "nd", "rd"][day % 10 - 1]