I'm not sure that you need to be more specific than just "Cocktails," but you definitely need the plural. You are not going to serve just one cocktail to the first person who gets to the right place, you are going to be serving many of them to all of the people who come to the event, even if they only drink one each.
Presumably, you are going to include the time and place where the cocktails will be served, but as a header for those details, "Cocktails" could be sufficient. You can be more expansive if you want to and it would be consistent with the rest of the printed agenda or program, following the suggestions of MBK and murciana.
Note also that if this is a conference of some kind and you expect the participants to pay for the cocktails, the usual AE expression would be "Cash bar." "Cocktails" or "Cocktail hour," etc., implies that the host is paying the bill.
If you're using it as a single word or as a noun preceded or followed by another phrase, use the plural: Cocktails / Break for cocktails / Cocktails and nibbles
If you're using it as an adjective, use the singular: Cocktail hour (probably the most common term, even if the time allotted is not exactly one hour) / Cocktail break / Cocktail party / Cocktail reception / Cocktail social / Cocktail get-together