The last day of your 2020 use year.
If you want a specific date, we would need to know your use year (month). The jpg doesn't give us that.
For example, if you have a September use year, your banked 2019 points would expire at the end of your 2020 use year, which is August 31, 2021, so that is the last night that could be paid for with the banked 2019 points.