Felicitous describes something that's pleasantly apt or fitting. Felicitous words you write on your friend's birthday card are the ones that perfectly suit the occasion and make her happy when she reads them.
Felicitous can mean "appropriate," but it also describes something that's lucky. When you plan a trip to the amusement park and it turns out that the sun is shining, that’s felicitous. If you need to mail a package by a certain date and you make it to the post office just in time, that’s also felicitous. The Latin root of felicitous is felix, "happy or lucky."