Release 7.1.0_GA_3140.UBUNTU10_64 UBUNTU10_64 NETWORK edition
Show Calendar Week is not available in the default class, so missing:
xttp://IP:7071/zimbraAdmin/ -> Configuration -> Class of Service -> default -> Preferences -> Calendar -> *Show Calendar Week*
Show Calendar Week is only shown in "Month view"/"mini calendar" in Ajax Web client, so not available in standard or mobile. Really unfortunate that it's no in the Month view of the mobile web client.
Can this be linked to a current bug?
There is also this bug "Calendar Week Numbers wrong when first day of week is not Monday" https://bugzilla.zimbra.com/show_bug.cgi?id=55174
If your in Europe it's probably Monday and that can be changed for all users in default class.