While I have had many 36 hole days much like the previous posters...Cypress/MPGC, Any 2 Bandon courses, etc.... Two days stand out the best to me.
Back in 1994, I was in the USAF in pilot training out in Texas. My roommate/ best friend and I went down to play The Quarry in San Antonio. After the round, we decided to go to the base golf course and play another 18. We both played great at each course which added to the enjoyment. From there we went and got crab legs, wings and beer. Left there, went to the Officer's Club, met some female officers in nursing school and the rest is history. As my friend and I have commented for the next 20 years, "THAT was a pretty good day"!
Recently, I had another great day at Yeamans Hall. We played 18, then the member said, "We can go in and have a sit-down lunch, or we could grab a sandwich and play another 18". Of course, we chose another 18. After our 36th hole, the member said, "Well, there is about 2 hours before it gets dark". Of course, we headed back to the 1st tee for another 9 holes!!! Finished the day with a beautiful sunset, ducks flying over and putting out on the 9th. Epic! Drove from there to Secession to do the same thing the next day!!!
I will never forget either day!!!