On March 15, 2017, Blue Corporation purchased 10% of the Gold Corporation stock outstanding. Blue Corporation purchased an additional 40% of the stock in Gold on October 24, 2017, and an additional 25% on April 4, 2018. On July 25, 2018, Blue Corporation purchased the remaining 25% of Gold Corporation stock outstanding. For purposes of the 338 election, on what date does a qualified stock purchase occur? What is the due date for making the 338 election?

Please show an explanation in detail.