By "Christian calender" do you mean the calendar, or the Christian holidays? Two different things, you know...and I think that, yes, knowing the calendar (as in 12 months, 30-31 days a month for the most part, seven days to the week, today's Nov. 10th, a Thursday, and get your ass to work) should be required for the citizenship exam.
I also think they should be required to understand that different people celebrate different holidays (so that some irate Arab isn't screaming obscenities at the local Safeway for being closed Christmas Day)...but that would disqualify most current citizens from citizenship. Which, upon reflection, is not necessarily all that bad an idea...