The correct sentence that shows how the apostrophe is used to form a contraction is:
"It’s going to be a great day for a picnic."
(Note: There is a typo in "picnic" as "picni" in your provided text.)
The other sentences have incorrect uses of contractions:
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"You’re car is parked in the wrong spot." should be "Your car is parked in the wrong spot." (You’re = You are, which is incorrect here; it should be your, indicating possession.)
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"They’re not be attending the meeting today." should be "They’re not going to be attending the meeting today." (They’re = They are. The current form is grammatically incorrect.)