I'm seeing questions about this, but no definitive answer, so let me ask:
Apparently, the night of April 15th counts. IOW, if my 10th night is Apr 15th/16th, I will get the bonus. However, what about a longer stay?
What if I have stay from perhaps Apr 13th to Apr 18th, and Apr 15th is my 10th night. In that case, does the Marriott system award me the bonus, or do I have to check out on the 16th for the system to recognize my nights as occurring within the bonus period?