Explanation:

- Option 1: "an" is used before words beginning with a vowel sound. "Long" does not start with a vowel sound, so this is not correct.

- Option 2: "one" specifies the number distinctly but doesn't fit naturally in the sentence structure for common usage in this context.

- Option 3: "a" is the most appropriate. It is an indefinite article used before words starting with a consonant sound and fits naturally here as it indicates a nonspecific weekend.

- Option 4: "the" is a definite article used for specific nouns. It suggests a specific long weekend already known to the speaker and listener, which doesn't fit here.