英文：星期几 前 一定要加 ON
China to announce recruitment plan for women astronauts
The Chinese government will announce a plan to choose the country's first female astronaut at the end of this year,the Beijing News reported Monday.
Taiwan mudslide may have buried 600 villagers
A mudslide triggered by torrential rains from a typhoon may have buried up to 600 villagers in mountainous southern Taiwan,disaster officials said on Monday.
为什么第一段结尾的Monday前没加ON,而第二段结尾的Monday前加了ON.区别是什么,什么情况下,星期几前可以不加ON.