A "red letter day" to me means having something significant happen, something usually good. Since coming to Bosnia, the origin of the word has become clear: it is the days in the Orthodox (and Catholic I hear) calendar marked in red ink: they are more significant, more special and need to be respected more. Here, you are supposed to fast and not do any significant work on that day. Putting on your washing machine is a complete no-no for instance...
And even after so many years I don't really know when they are going to happen, and even when I had a calendar in my house, I would still forget to look and yip, put the machine on! Last week, I was reminded again of this facility of mine to use the washing machine on days that are marked with red ink: Thursday was "Blagovjesti" (Blessed news): the day when the Angel Gabriel came to tell Mary she was going to give birth to Jesus. And I was busy all day except for 2 hours in which I fatally decided to put the washing machine on.
When I went onto the balcony to hang out the washing, my landlady was quite nice about it, but in no uncertain terms let me know what I had done... she even pointed that I was probably the only person in Banja Luka who had put my machine on, as it really was a special "red letter day": more special than most.
It is mostly a superstition - i.e. if you put the machine on, something bad will happen - and as I don't believe it, I don't stress about it like other people! But I hope one day I will learn when not to put the washing machine on and not mess up on a red letter day!
In other news... this week has been a red letter week in the Chaplin family around the world: my brother (in Canada) managed to separate his shoulder, and my mom (in South Africa) broke her leg! I am hoping that the third thing is that my parents' kombi broke down... otherwise who knows what could happen. (good thing I am not superstitious!)