[from my SPOT THE ERROR archive]
Many second language learners are confused with the usage of "-st", "-nd", "-rd" and "-th" when it comes to writing dates, as seen in the above banner and bunting. They would rather look at the number and not how the date is read or how it is pronounced.
For those who are not so sure, this is how it works:
ST - first
ND - second
RD - third
TH - fourth/fifth/ sixth etc.
1st January is read as the first of January so you cannot write it as 11st of January as it is read as the eleventh January. The correct one will be 11th January.
31st January is read as thirty first of January so you cannot write it as the 31th January as you will not be able to read/pronounce a date written in that manner.
Hope this will help.