The unfortunate March 22nd 2017 terrorist incident in London occurred one year after the 2016 Brussels bombings, which brings about some questions: Are terrorist attacks more likely to occur on anniversaries of previous incidents? What about on holidays or other significant dates?
The RAND Corporation recently conducted a study on this topic and found that "the days around Independence Day (July 2 and 4) and New Year’s Eve (December 31) each experienced an unusually large number of attacks". As a counterpoint, they did not find an increase in victims on the anniversary of September 11th, a significant date for Islamic terrorists, though they did find an increase in the number of attacks planned for September 11th. In pure speculation one could assume that security and intelligence forces are more vigilant leading up to and on September 11th, possibly reducing terrorist success rate. The RAND study went on to conclude "there is no evidence that terrorism has occurred more regularly on dates perceived to be symbolically significant. When a terrorist event happens in a major city, there is no evidence that another event will happen in that city (or even elsewhere in the West) in the days or weeks afterward".
RAND also found that general terrorist behavior does change over time, so in the future terrorists may be more or less inclined to target specific dates. We do recommend researching and being aware of terrorist anniversaries and other symbolic dates such as national or religious holidays when conducting risk assessments, though the weight assigned to the data must be considered carefully.