Answer:
the celebratory dinner
Explanation:
the sentence is basically about the dinner, and roast beef & mashed potatoes is what the subject (the celebratory dinner) <em>is.</em>
The conventional rule is to take the plural form of the verb when two nouns are joined by the conjunction 'and'. However, there are exceptions. Only then can you break the rule.
When the two nouns refer to one thing only, then you can use the single form. For example: The president and the CEO joins the meeting today. In this example, the president and CEO apply to one person only. Hence, you use the singular form of the verb.
