Assume you are talking about your lawn...

Fertilize once the grass actually starts growing. I wouldn't be too quick about it though. We could go back to real temps anytime.

I noticed my grass greening up just today. I fertilized in the fall.

Weed killer is also when weeds are actively growing. However, if you are referring to the Crab Pre-emergent stuff, (Step 1 if you do steps), do that when the forsythia in your area are blooming.

Just an opinion to consider . . .

Much non-point source pollution comes from yard run-off. Often lawns don't really need weed and feed (I think we've maybe used it twice in 15 years.) Scott's and other similar companies would like you to think that it is needed so they can sell more. Mostly DH just mows high, lets the clippings get mulched right into the lawn (sometimes he double cuts to make the clippings finer), and chops the leaves up with the lawn mower in the fall rather than raking. This combination helps decrease the sunlight getting to weed seeds and allows slow breakdown of organic matter which helps the soil texture and adds nutrients slowly. It also decreases the amount of water needed since there is more organic matter in the soil and the soil is more shaded. A soil test will tell you if you need fertilizer (or lime, which in New England's mostly acid soils will make several nutrients more available.)

So more often than commercial vendors of lawn treatments would like, the answer to time of application might be, not this year.