Datas: "in" ou "on"?

Use "in" quando mencionar apenas o mês, ou apenas o ano, ou o mês e o ano juntos, sem o dia.
Exemplos:
My birthday is in July.
Christmas is in December.
I'm going to visit my cousins in October.
The company was founded in March 1972.


Use "on" quando mencionar somente o dia, ou o dia junto com o mês/ano.
Exemplos:
The party will be on the 12th.
My birthday is on July 15th.
Christmas is on December 25.
I'm going to visit my cousins on October 31st.
The event will be held in New York on April 10, 2011.