Occasional poetry is simply poetry written for an event or occasion. My grandmother Carrie Strittmatter Dotson wrote a Christmas poem every year. Here is the link to an article about poet Elizabeth Alexander's thoughts about writing an occasional poem for Barack Obama's inauguration:
NY Times: "Mr. Obama’s inauguration, on Jan. 20, calls for an 'occasional poem,' written to commemorate a specific event. This is not precisely what Ms. Alexander does, but she is preparing for the challenge.
“'Writing an occasional poem has to attend to the moment itself,' she said in an interview, 'but what you hope for, as an artist, is to create something that has integrity and life that goes beyond the moment.'”