You will need to get from Lisbon Airport to Lisbon Sete Rios Coach Station from where the buses depart.
This means either a cab or the Aerobus service.
See this link on my blog for more info:
This means you probably will not get to the bus station until at least 17:30.
Rede Expressos do not yet have the timetable online for May, but based on the current timetable there are departures at 18:30, 19:15, 19:30 and 20:00.
Transit times are between 1,5 and 2,5 hours depending on the service.
Suggest that you check the times closer to the date on there website: