Refers to someone who says they will start their diet next week, tomorrow or a specified date.
Example 1
Person 1: Here have a cupcake
Person 2: I just started my diet, but it's ok I'll start next Monday

Example 2
Person 1: I'm getting fat, I'm going to start my new diet as of tomorrow
Person 2: You said that last week. Is that the Groundhog Diet?
by rr11 May 22, 2014
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