Q:

How often should I fertilize my lawn?

How often should I fertilize my lawn? Should I follow a certain schedule? I have heard of people fertilizing on around holidays [US: Easter (sometime in April), Memorial day (last week of May), fourth of July, Labor day (first week of September)].  

A:

My decades ago hort 101 class said fertilizer is not really necessary if you use a mulching mower.  If you bag the clippings or if you want thicker greener lawn you can apply fertilizer up to 3 times per year.  The applications should be timed to catch the grass growing but not the weeds.
Here in central Iowa with bluegrass/ryegrass/fescue lawns the grasses go brown/dormant during the hot part of the summer (July, August) and grow most vigorously in the cooler spring and fall.  The weeds go nuts in the spring (April, May, June) - we do not want to encourage that!  So the fertilization schedule works out to once in the first 1-2 weeks in September, again 2-3 weeks after that and (very optionally!) once in the early spring just after the ground thaws (first 1-2 weeks in April).

A:

Below are a few quotes from this answer I posted here on SE. Hopefully you find them of some benefit:

Seeing as my lawnmower is a mulching mowing, I leave all the grass clippings on the lawn (free, natural fertilizer), except with first cut and last cut of the season. I collect up those cuttings and dispose of them via a community yard waste pile.
Fertilize the lawn with corn gluten meal (by Bradfield because I can get it easily locally) in very early Spring, when I see the Forsythia shrub in flower in my area.
For the first time this year I also decided to fertilize the lawn with corn gluten meal in mid June. Why? See "Table 4" here: Cool-season grasses: Application schedule for organic fertilizers
Corn gluten meal isn't a "magic bullet" for controlling unwanted weeds (plants). I've read and been told that it can take at least 2 to 3 years (following recommended application rates on yearly bases) before seeing any results with this method.
I make 5 gallons of compost tea each week (from late Spring to earlier Autumn "Fall") and apply the 5 gallon batch to the front garden one week, then the following week apply a new fresh 5 gallon batch to the back garden. I repeat that cycle for the period given previously. I have been doing this for 2 years now, and without question I have noticed a massive increase in worm activity eg lots of worm castings on the surface of the soil.
I spread the 5 gallons of Compost Tea via a watering-can over approx 1800ft² (170m²). Lawn area is about the same front & back for me. So one week the front lawn gets treated, then the following week the back lawn gets treated, repeat, repeat...
Around "Labor Day" (beginning of September, early Autumn "Fall") here in the US, I prepare any bare spots for reseeding. Reseed using an appropriate seed for my lawn type. Cover the whole lawn with ½ to 1 inch (12.5 to 25mm) thick layer of STA-certified compost (bought in bulk locally). Water as needed, ie Amount needed for good germination to take place.
Fertilize the lawn with Ringer Lawn Restore (by Woodstream Corp) at the end of September, early October.

You may also find the actual answer I pointed to above of some help:
What's an organic way to discourage crabgrass from a large “lawn”?

A:

In upstate NY here - I usually fertilize mine once in Spring (late in May) once in the summer (around mid-July) and once in the Fall (around mid-Sept) and it seems to do the trick just fine.

