What is in your Tools > Preferences > Database > NLS > Timestamp Format field?
Mine contained DD-MON-RRRR HH.MI.SSXFF AM and the SQL History timestamp reflected that format. Then I changed it to
DAY-DD-MON-RRRR HH.MI.SSXFF AM, closed and re-opened the SQL History tab, and the day of the week displayed ahead of the regular timestamp...
Mine was DD-MON-RRRR HH24.MI.SSXFF3 changing it to DD-MON-RRRR HH.MI.SSXFF AM then back fixed it for me.