I am looking for a particular word here. I heard this somewhere but just don't remember. gr8ly appreciate help. thanks.

Let's celebrate this ____________ occassion. (hint: It starts with a 'h'.)

What does the word "cross-listed" mean?

Thanks again.
How about "happy"?

Never heard of "cross-listed", but it sounds like "cross-referenced", which usually refers to redundant indexing in a publication, which is a good thing. For example, in a cross-referenced Encyclopedia, the entry for Civil Disobedience might say "see Gandhi", and the entry for Gandhi might say "see Civil Disobedience ."
No, not happy. It was something else. Please help.
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Cross-listed would probably mean listed in more than one place, document or file.
adjectives you could put there

do you know how to use a thesaurus - try this one http://www.visualthesaurus.com/online/index.html
Yeah, I think it's "historic". What does it mean exactly? Thanks.
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It probably means an occasion of great significance, an occasion which might be remembered in years to come. In one sense it will become "history" - part of the story of the past. It can also be used in a slightly joking way, when the event is REALLY quite trivial.