Normally, we test for anticipation effects as an assumption when using difference-in-differences. However, it seems that it is necessary for examining the impact of laws on firms' behaviour. For example, we want to test if anti-collusion laws affect firms' asset growth. Therefore, we expect that firms will know about the establishment of the laws (due to media, government discussion, insider information), leading to firms changing their asset growth before the actual event dates.
On the other hand, I am wondering if we need to test for anticipatory effects if the event date is a natural event (i.e., not a law, etc.), say for example, that the level/percentage of vaccinated people/population is 30% or a tsunami. In my opinion, I do not think a formal anticipation test is needed here. Is this correct?