Short answer
Holes are the symptom where the first question is not what to do but what made them, because the neatest holes in a pollinator garden are cut by a bee worth keeping. Look at the shape before anything else. Smooth circles taken from the leaf edge are leafcutter bees and need nothing at all. Ragged holes with dark droppings on the leaves below are caterpillars, which on a host plant is the point. Lacy leaves with the veins left standing are Japanese beetles, and slugs leave mucus trails and work overnight.
Listed in the order they are worth ruling out. It is normal for more than one of these to be happening at once, so work down the list rather than stopping at the first match.
First on the list because it is the one cause here that asks nothing of you, and because the cut is unmistakable once you know the shape. A leafcutter bee takes neat discs from the edge of a leaf to build her nest.
What to check first
Look at the shape of what is missing rather than at the plant as a whole. Smooth, near-circular pieces taken out of the leaf edge, never from the middle of the leaf, is this and nothing else on this list.
Ragged holes or whole sections eaten in from the edge, with dark droppings on the leaves below. Ranked second because on a plant that hosts butterfly or moth larvae this feeding is what the plant is there for, and that is worth establishing before anything else.
What to check first
Turn over two or three leaves near the damage and look along the underside and the midrib, then check the leaves directly beneath for small dark droppings. The droppings stay put even after the caterpillar has moved on.
Large ragged holes that were not there yesterday, worst in damp spells and on the leaves closest to the ground. The animal is almost never present when you look, so what identifies it is the trail it leaves behind.
What to check first
Look for dried mucus trails, silvery lines across the leaves, stems or soil nearby. Extension guidance calls these the most characteristic sign of their presence, and they last long after the slug has gone.
Dozens of very small round pits rather than a few large holes. The size and the sheer number are the identification, and the beetles give themselves away the moment the plant is disturbed.
What to check first
Brush a hand across the foliage and watch what happens. Flea beetles are small beetles that jump, and they scatter instantly, which nothing else on this list does.
Leaf tissue eaten away between the veins so what is left looks like lace, with several metallic beetles feeding together in full daylight in July or August. Unlike everything above, you will usually see the insect itself.
What to check first
Hold a damaged leaf up to the light. Veins intact with the tissue between them gone, in a lace pattern, and beetles visibly on the plant right now, settles it.
Still not sure
This is exactly what your county Extension office answers, for free. They are the local horticulture experts for your area, they know what is going around this season where you live, and they will look at a photo.
Find your local Extension officePollinator Patch gives you watering and seasonal care for the plants you already own, written for your region rather than for gardening in general.