QUESTION:
When would a leapyear be for a mars calandar with 15 months and does
Mars even have a leapyear.

ANSWER from Smart Filter Jos Wennmacker on September 21, 1998:
From various  sources we  find that  the Martian  year is 686.98 Earth
days.  The Mars solar day (sol)  is considered to have a length of  24
hours, 39 minutes, and 35.25  seconds ( 24,65979 days); it  is usually
abbreviated as sol.  So the Martian year has (686.98 * 24) /  24,65979
= 668.59937 Martian days.   If you have  15 months in a  Martian year,
the number of  days in a  month would be:   668.59937 /  15 = 44.57...
days.  Oops, impossible!  So  most months will have 46 days,  and some
46 days. 15 * 44 = 660 so we need 7 months with 44 days, and 8  months
with 45 days, because 7 * 44 + 8 * 45 = 668 days.

This means however, that we miss  0.59937 days every year, or about  6
every 10 years.  So  we have to devellop a  scheme in which we have  6
leap years in 10 years.  It would  be best to fix a scheme that is  as
regular as posible, for instance:  n l n l l l n l n l with n being an
normal year, and l being a leap year.  But:  l l n l l n l l n n would
also be a good scheme.  The leap day has also to be set by  agreement.
For instance the second month of the year has an extra day, or in leap
years there could be an extra  day, not belonging to a special  month,
but just an extra bank holiday!

This scheme would  not be perfect,  because we insert  (6 - 5.9937)  =
0.0063 days too many in ten years.  So once every 1587 years (10* (1 /
0.0063 = 158.7)), for simplicity let's do it every 1600 years we leave
one leap year out.

Of course, we should not use the siderial Martian year, but a synodial
but I do not have data about that.