Depends on where you live and your grass.

Fertilizer: Generally for northern type grasses you should fertilize on Memorial Day, Labor Day, and again when the grass stops growing but before it goes dormant. This might be in October, November, or December depending on your lawn and location.

Herbicide: Spot spray broadleaf weeds with something like Weed-b-Gon or Weed-b-Gon Chickweed, Clover, and Oxalis Killer. There are some spring weeds which germinate in the late summer with the late summer rains and immediately go dormant until spring. During those rains would be a good time to apply a preemergent herbicide.

Grub treatment: Grubs are treated in June and only after you have found uncountable masses of grubs. Most lawns can tolerate a large number of grubs without showing any symptoms, but a huge population should be treated in June or July. If you see grub damage this time of year, you missed your opportunity to stop the damage.