In electional astrology, an event chart is a horoscope that is cast for the date, time and place of a particular event.
Such a chart is interpreted to gain insight into influences surrounding the event and an outlook for possible developments stemming from that event.
For example, if a chart was cast for the date of one's marriage then the chart would give an indication of what the couple can expect during their married life.
If a chart is cast for a national event then one can interpret the chart to determine influences surrounding the event.
This is in contrast to a birth chart, which is calculated for the actual time of a person's birth, but the design and interpretation are similar.