OK, figured it out.
Open Apple Calendar
1) Click on File/Export/Export
2) Nagivate to Chosen folder, or create one "Calendar Exchange" works
3) Name your iCal file (Work, Personal, Birthdays......) for saving, i.e. "Apple Work"
4) Press Export button. Creates file "Apple Work.ics" in folder Calendar Exchange.
5) Repeat for other calendars (Work, Personal, Birthdays......)
Open SeaMonkey Calendar
1) Click on Events and Tasks/Export
2) Select calendar to export (Work, Personal, Birthdays......). I use the same names as in Apple Calendar, i.e. "Lightening Work".
3) Repeat for other calendars (Work, Personal, Birthdays......)
You now have a folder CALENDAR EXCHANGE with parallel files APPLE WORK.ics, LIGHTENING WORK.ics, etc.
To read an Apple calendar into Seamonkey /LIghtening, click on Click on Events and Tasks/Import and follow the instructions.
To read a Seamonkey/Lightening calendar into Apple Calendar, click onFile/Import /Import and follow the instructions.
While this is not automatic, it does work. ou can email the .ics files to others for them to import.