When the high tide is very high, and the low tide is very low, it is a spring tide. On which days did the observer experience a spring tide?

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.