The days allowed by custom for loading or unloading a vessel. Failure to complete the job within these days may make the guilty party liable to pay demurrage. Lay days may be counted in working days,running days, or weather working days. Where the number of lay days is not specified in the contract, the work must be done in a reasonable time (this might allow for delays due to strikes. etc.).
Reference: The Penguin Business Dictionary , 3rd edt.