preposition
1
BAD : On last Monday we went to the Railway Museum.
GOOD : Last Monday we went to the Railway Museum.
BAD : On every Saturday morning we go shopping.
GOOD : Every Saturday morning we go shopping.
2
BAD : I was surprised to see your picture on the newspaper.
GOOD : I was surprised to see your picture in the newspaper.
◆◆◆
You see a report, advertisement, photograph, etc in a newspaper or magazine (NOT on ): 'I came across the article in this month's edition of Woman's World.'