‘To make someone’s day’ means to make someone very happy.
For example:
Brian received a letter from the tax office notifying him that he would be getting a rebate and it really made his day.
or
Mary’s crush surprised her with flowers and chocolates and he made her day.
In the meme, one rotation of the earth is by definition a day so it would ‘make a day’; the joke is the combination of the literal meaning of what defines a day and the expression to make someone very happy.
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