Selling your unit? Even if you won't be the owner for the entire month, your full monthly assessment is due on the 1st of the month in which you sell.
The pro-rated portion of your assessment, for the days during which you are no longer the owner, will be recovered from the buyer at closing. Your board or management company must issue a Paid Assessment Letter in order for you to close, so be sure your assessments are all paid up prior to your sale.