There is a bug report.
I went with this solution.
Modified from the above site:
- date formatscd /usr/share/plasma/plasmoids/org.kde.plasma.digitalclock/contents/ui/
cp DigitalClock.qml to DigitalClock.qml.install
nano DigitalClock.qml
Find the following line:
dateLabel.text = Qt.formatDate(main.currentTime, main.dateFormat);
Replace it by the following:
dateLabel.text = Qt.formatDate(main.currentTime, "ddd, MMM dd, yyyy" );