You can say "When is a good time to have a party" or "When is the right time to have a party". The, a, and an are articles, used to refer to nouns. The is known as a definite article, used to refer to a noun where the noun is a specific member of a group (a definite noun), such as "The coffee I drank this morning was bitter". It is specifically the coffee I had for breakfast which was bitter. In your example, you say "When is the right time to have the party" because the term "right time" implies a specific correct time. By contrast, a or an (known as indefinite articles) are used for general nouns (indefinite nouns) where any member of a group is referred to. For example, "He drinks a cup of coffee with his breakfast every morning". Here, the specific cup of coffee is not important, only the fact that he drinks coffee each morning is important. In your example, "When is a good time to have a party" uses the indefinite article "a" because the noun "good time" is not specific enough to warrant the definite article "the". The term "good time" may refer to a number of possible times. In contrast, we say "The best time", not "A best time", because the noun "best time" implies a specific single time which is the best. I hope this makes sense and was useful. In the example you gave it is not really obvious why "the good time" or "a right time" cannot be used. Another case where you simply have to memorize the usage, even if it doesn't make perfect sense. In other examples, such as "I read a book" (this could be any book) and "I read the book" (referring to a specific book) it is more obvious why "a" and "the" are used. This is another example illustrating why you need to be exposed to the language as much as possible, because you will never hear a native speaker saying "choose the good time", it is always "choose a good time". Similarly, you will never hear a native speaker saying "choose a right time", it is always "choose the right time".

Why do we use 'the' when we use the adjective 'right'...

"When is a good time to have the party?" or "When is the right time to have the party?"