"Tijdens het festival heeft het de hele tijd geregend."
Translation:During the festival, it rained the whole time.
Could this also mean "It has rained all the time during the festival"?, as in we are still at the festival and it has been raining all the time?
No, where English uses the simple past tense for actions that have ended, Dutch generally uses the present perfect tense. In English, for an action that is ongoing you would use "It has been..." So "During the festival it has been raining the whole time (or: all the time)" in Dutch would be "Tijdens het festival regent het al de hele tijd." What makes the difference is that little word "al" in Dutch.