Much like Martin O'Malley's campaign for President, we know that there will be a special election in Ward 2, but we don't know when.
It all depends on when the seated Alderman formally hands in his resignation to the mayor. Once the mayor receives such a document, she has 5 days to announce when the special primary election will be. The primary must be no sooner than 23 and no longer than 30 days after the formal resignation of Mr. Christman.
The general election must occur no fewer than 21 and no longer than 30 days after the primary.
The when of this whole deal is an interesting proposition. Mr. Christman and Mayor Moyer each have sufficient power to affect who wins the special election, even though neither of them will be running. Specifically, each has the power to determine--within the guidelines of the charter--when the elections will take place. If it were to take place during the holidays, which is a distinct possibility, voter turnout will likely be lower than normal. Whether this favors a Republican or Democrat is probably debatable, but certainly interesting.
We can pontificate together on the political ramifications of the election timing once we know the actual dates.