Special assessments that are voted on by the board and charged to unit owners become the responsibility of the unit owner when they are charged, despite when they are due to the association. Your board may allow for payment of the special assessment by installment over time, but the entire amount charged is added to your account at the time the special assessment is implemented.
This means that if you sell your unit before all installments come due, the entire amount may be collectible at closing. An exception may apply in the case of a special assessment that is financed by the association for an extended period of time and charged monthly to the unit. In that case, the special may be assumed by the buyer when purchasing the unit (depending on how the sale is negotiated).