Some synonyms of symetrical are: balanced, commensurate, equal, proportional, regular, shapely, commensurable.

acceptable, agreeable, all right*, ample, aplenty, appreciate, comfortable, commensurable, commensurate, common, competent, copious, decent, due, galore, pleasing, plenteous, plentiful, plenty, proportionate, satisfactory, sufficing, tolerable, unexceptionable, unexceptional, unobjectionable. these are all synonyms for the word sufficiently. .

Involving or exhibiting symmetry; proportional in parts; having its parts in due proportion as to dimensions; as, a symmetrical body or building., Having the organs or parts of one side corresponding with those of the other; having the parts in two or more series of organs the same in number; exhibiting a symmetry. See Symmetry, 2., Having an equal number of parts in the successive circles of floral organs; -- said of flowers., Having a likeness in the form and size of floral organs of the same kind; regular., Having a common measure; commensurable., Having corresponding parts or relations.

Involving or exhibiting symmetry; proportional in parts; having its parts in due proportion as to dimensions; as, a symmetrical body or building., Having the organs or parts of one side corresponding with those of the other; having the parts in two or more series of organs the same in number; exhibiting a symmetry. See Symmetry, 2., Having an equal number of parts in the successive circles of floral organs; -- said of flowers., Having a likeness in the form and size of floral organs of the same kind; regular., Having a common measure; commensurable., Having corresponding parts or relations.

Because Our calendar is a product of long evolution from a hunting society to an agricultural society. The hunters originally used a lunar calendar--new month each new moon. But new moons occur every 29.5 days, which is 12 in 355 days. This is not commensurable with a seasonal calendar such as agricultural societies would want. Ancient Rome tried to fix this by having a 13th month every third year. This did not work out too well, because the head priests played games, calling for an extra month whenever people they liked were elected to head the government (2 year terms). Julius Caesar hired an ethnic Greek from Alexandria Egypt, Sosigenes, to create the 365 day calendar with an extra day every fourth year.
12 month calendars are favored because business works on a quarter system.