Saturday AM is easiest! Your biggest hold up will be rental car agency in NYC for those which didn't pick up on Friday night - they will be there Saturday AM in line.

Drive from Manhattan (verify you take Holland Tunnel to Rt 1/9 South (stay left after exiting tunnel and remain left hand side) - don't follow GPS requesting you take NJ Turnpike or signs which say EWR Airport to the right via 78 <both roads meet again and you end up back on Rt 1/9 before hitting airport> - and it should take you maximum 25-30 minutes.

You can fill up on gas (w/receipt) as soon as you exit Tunnel as airport is less than 13 miles away to avoid more inconvenient attempt to find gas or over priced gas in EWR.

As you probably know drop off ALL passengers/bags at terminal before you return the car. Not sure of status of AirTrain but tell your party you'll meet them on the other side of security as you will take a while (if waiting for shuttle bus). Consider walking from rental car to terminal if you are at A or B.