If covered calls are written, the HOLDER of the calls who bought the calls can exercise them anythime they want, up to and including exiration day. If the covered calls are exercised before the stock goes exdividend then the person exercising the options early wil get the shares and the dividend. He willl lose the shares at the strike price and not get the dividend.
Many people think that they can write a call option and depend on a share price drop at exdividend. They don't realize that their is the possiblity of the call holder exercising early. An early exercise is quite common for Intel options.
It is not a big deal even if the shares are exercised early as long as the call writer understands shares are (depending on strike and month) likely to be called.