The readily observable date on which work commences on a construction project.This date is important in the context of mechanics'and materialmen's liens for unpaid construction bills, because the date of the lien is effectively backdated to the date of commencement of work,even though there may not have been a failure to pay bills until many months later.In many states,if there is a lien filed,and the commencement of work occurred before the construction mortgage was recorded,then the lienholder is in a first lien position and the construction lender is in second place,comparable to having a second mortgage rather than a first mortgage.