In my experience, toward is American English usage
and towards is British English. The terms are seldom
intermingled within American English or British English.
Or are there special occasions where, say, towards
might be used in American English?
Barry is correct in identifying toward as principally
American English and towards as principally British
This difference is also noted in Quirk, Celce-Murcia, The Collins COBUILD,
L.G. Alexander, and various dictionaries.
Quirk (A Comprehensive Grammar of the English Language, Longman,
1985) notes that the -ward/-wards distinction
is also true for words such as "eastward/eastwards" and "upward/upwards."
An interesting caveat: The New York Times Manual of Style and Usage,
p. 335, gives this entry for toward: "toward (not
towards)". No further description.