The timing of this one is interesting, and I suspect not what they were initially planning to do. My bet is that they were probably initially planning to set these off on Thursday or Friday, but moved it up because of the recent capture of some of the prominent ringleaders in the area. I hate to say it, horrible as these events are, but I have a sinking feeling that it might have been much, much worse if the French authorities hadn't recently captured some of these people. (And, God help Belgium and rest of the Low Countries, it is possible that it still could be if they have missed more of them.)
Why? Because in that part of Europe, the Thursday and Friday before Easter (Holy Thursday and Good Friday), are some of the busiest travel days of the year, very similar to the way that the day before Thanksgiving is in the US. In the UK and Ireland, Easter is the most popular holiday for going home to visit your parents, so the trains and planes are full of those folks making their way home from jobs in Europe to visit their folks. (Also, if you are fighting a holy war, it has double significance as the most sacred of Christian holidays.)