Early or late summer is a good time to smooth out those holes, dips or bumps that find their way into your yard. They occur for a variety of reasons. Holes can be caused by rodents, kids, dogs, cats and misused equipment. They can usually be filled with good native soil or a planting mix. If the areas are small, the grass will fill in. If larger, they should be seeded or sodded over. Take whatever steps necessary or possible to prevent continued or future damage.
Depressions from settling soil can be trickier to repair. You first need to determine what caused the soil to sink and remedy the situation. If more than an inch or so deep, you should remove the sod around the affected area; correct the cause and then backfill to slightly above level to allow for settling. If the removed sod is in good condition, it can be set back in place. If it is not, resod or seed the area. For small depressions, or even those over a larger area that are less than an inch deep, you can sprinkle soil over the area about a quarter inch at a time. Keep this up every two or three weeks until the depression is filled in. To help improve a heavy clay soil, a peat, sand, compost mix makes a good top dressing. Use about 40% peat, 40% compost, and 20% sand. Good topsoil also works well.
When bumps occur in the lawn, you need to determine how they got there. If rodents are the cause, you need to correct that problem first. Get rid of the rodents with traps or controls labeled for that pest. You may end up with a depression when you push down on the bump and will then follow the steps above.
It is best to take these steps when the lawn is most actively growing. This helps it fill in more quickly. Avoid disturbing the sod in the middle of the summer when the grass may be already struggling.