Answer:
On full moon or new moon days.
Explanation:
High tide always happens on the days of the new or full moon. This is because on these days the moon exerts a gravitational force directed in the same direction as the lunar direction, which is present directly at tides. On the other hand, during the days of a crescent or waning moon it is possible to observe the low tides, because on these days the gravitational force of the moon is greatly reduced.
