AT

at a specific time: at 7:20, at dawn, at dinner

ON

on a specific day or date: on Tuesday, on June 4

IN

in a part of a 24-hour period: in the afternoon, in the daytime [but at night]

in a year or month: in 1999, in July

in a period of time: finished in three hours

AT

at a meeting place or location: at home, at the club

at the edge of something: sitting at the desk

at the corner of something: turning at the intersection

at a target: throwing the snowball at Lucy

ON

on a surface: placed on the table, hanging on the wall

on a street: the house on Spring Street

on an electronic medium: on television, on the Internet

IN

in an enclosed space: in the garage, in an envelope

in a geographic location: in San Diego, in Texas

in a print medium: in a book, in a magazine