The 24-hour format reading for 00:00 is "zero hundred" or formally "zero hundred hours." However, this is primarily used in military, aviation, or technical contexts. In everyday American and British English, people almost always use the 12-hour clock. For 00:00 in daily conversation, everyone would simply say "midnight." You can also say "twelve AM." The word "tonight" provides perfect context if you say something like, "The party tonight starts at twelve."