I don't know how many trays you have, but one option would be to do a Home Depot run and pick up an inexpensive irrigation timer ($20) and a run of drip tubing and emitters. Set an emitter or two to dump into each tray and then turn the water on.

If the 18 plants fit into a single tray, you can just drop the hose end into the tray and forget about tubing and emitters.

Time how long it takes to get filled to the point where you normally water and then set the timer for that amount of time and the X times per week you need to (or think you need to) water.

It would be terrible to hope that they were sufficiently watered and find you were wrong.

Andrew

I don't think 4 or 5 days is going to hurt anything. Plants experience a little drought and are fine and I really doubt they will completely dry out in that amount of time. You'll only be skipping 1 watering with the schedule you stated. Just water heavy right before you leave. Maybe even dip the peat pots in water for a little bit to absorb and stay wet.

Ask an extended family member or a neighbor you trust to water them while you're gone. If you ask a neighbor, you may want to compensate them in some way.