Just about every website I know suffers seasonal and habitual visit changes You can usually confirm this by comparing year-to-year (assuming you have these stats available) Equally, if you noticed (or can check for) a reduction over previous bank holidays (such as Christmas and New Year) then this is most likely the cause Ironically one of my own websites has the opposite issue It has more visitors in the four weeks leading up to holidays, a consequence of that is that during the holiday the number of visitors can fall by about 50%, but this usually returns to previous levels after the holiday This is quite typical for a travel / holiday website
Before assuming anything just check your keywords ranking. Is there any drop in ranking of your website's keyword? If not then may be due to Easter.