I saw where Robin had these answers for a couple of different questions:
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You can use whatever you like to "mark" your card. Previews for that day's freebie, a screenshot of your post # in each day's thread, your layout, etc. The card itself is just a quick and easy visual reference for me when I go through in March to give that sweet prize away!
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As long as your card is posted and your post is updated with your participation in at least 25 of 28 challenges by March 2nd at 11:59pm EST, you'll be entered to win our grand prize!
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It was in this post.