In this context (at a restaurant), it's very clear which you mean. In fact, you would say "What are the specials today?" or "Are there any specials today?", and if you want to order one, you say, "I'll have the special". Notice that we are talking about "the special"/"the specials" -- at a restaurant, we use the noun "special", not the adjective "special": "I would like to order the special" ( = noun) vs. "This rare diamond is very special" ( = adjective)

What is 'on' special today. What is at a discount rate today, "On" special, same at the supermarket. What is special today? Today is my birthday/today the sunrise was beautiful/today I do..... etc.

The word "special" has two meanings. When used as an adjective in a sentence, it means "something that is not usually available." For example, "This car has several special safety features." And when it's a noun in a sentence, it means the question you asked about the restaurant For example, "There's a special on coffee today."

Most restaurants in the UK will have a 'specials board' on the wall and and often a note on the menu. The waiter or waitress will also tell you what is on the specials list for the day. You should ask ''What is the special?' and not 'what is special ?' :)