Why do all good fares good only from 10 April onward???

[Brian Huether]

If you go to travelocity and do a search on Boston to Munich (or anywhere else in Germany, etc), you will note that all the good fares have either 31 Mar or 10 April as the latest departure date. I can not figure out why this is the case. What is so damn magic about April 10th and Mar 31? I am wondering if there is reason to suspect that that date will get pushed out as weeks go by (ideally I need to buy tickets for April 26 - May 2), or whether I should give up on that thought and just go with the good fares (e.g. April 10 - 17)

thanks,

brian