For something that isn't even a recognized BJCP style, Session IPA really does get around! This time of year, I'm always looking for good beers to take to the beach, and I don't know that anything fits the bill better than a good Session IPA.
Light in body, bright in flavor, and surprisingly approachable (for something with "IPA" in its name), it's a darned-near ideal warm weather, happy-hour-through-dinner easy-drinking beer. Brewing a keg of this to have on hand throughout the summer will be a good investment of your brewing time, and they're actually a bit more complicated to make than we might assume!
This isn't just a question of reducing the gravity (although that's one way to go, and we will be): it's a question of generating a lot of flavor from hops and finding light ways to balance those flavors. Otherwise, you're really just brewing an American Pale Ale. Let's crank up the AC and get to work, shall we?
As I mentioned earlier, there aren't guidelines for Session IPA, per se. We can, though, make a number of useful assumptions based on what the guidelines tell us about session-strength beers in general (which identifies our ABV range), and specifically what Session IPA is in relation to American Pale Ale. To quote the one and only mention of Session IPA in the 2015 BJCP Guidelines (from the American Pale Ale entry): "More balanced and drinkable, and less intensely hop-focused and bitter than session-strength American IPAs (aka Session IPAs)."
So, take an American Pale Ale, lower its ABV to about four percent, make it more bitter and hops-focused, and you have a Session IPA. Easy, right? Well, yes and no. The target is surprisingly clear for something that isn't even fully described as a style, but hitting it requires us to get creative with our grist. The hopping is the easy part, but if you try to go too simple with your grains you'll end up with a weak, naked-feeling flavor profile that's more like heavily-hopped pale malt liquor than session-strength IPA.
A first pass at this recipe was a disaster for me. I thought, "OK, let's keep the grist nice and clean," and I used mostly American two-row and some Cara-Pils and Victory. The bitterness was too harsh, the grain flavors disappeared, and it was drinkable but really, really boring. Well, if clean wasn't working, why not dirty it up some? I ended up with this, and haven't looked back since.
Start with five pounds (2.3 kg) of Maris Otter and one pound (half a kilo) of Vienna as a base (about as far from "all two-row" as you can get). To that, we add a pound of Munich and a pound of Crystal 20, which will bump up the melanoidins and add some light caramel sweetness (as well as adding some non-fermentable sugars, for body and texture). Finally, a half-pound (quarter kilo) of flaked oats will soften up the palate without adding so much protein that you think it's a mini-NEIPA. This should get you to about 1.044, or 4.3% ABV.
Hopping, as I said, is the easy part here. One ounce (28g) of Simcoe at the top of the boil should net you about 35 IBUs, and then add another ounce with five minutes to go in the boil. At flame out or in the whirlpool, double that amount and put two ounces (57g) of Amarillo in, and you're home free. I don't dry hop this beer - I just don't like how the resin feels/tastes in a low-ABV IPA.
Finally, I like my London Ale III (Wyeast 1318) yeast for this beer. The berry esters are a nice complement to the stone fruit flavors in the late hops, and not being one of the world's great attenuators isn't a big deal in a session beer.
Mash, lauter, sparge, and boil as usual, though if you want to mash a bit higher (one or two degrees) in temperature to try to increase the body a bit, it won't do any harm! If you can, though, wait to add your Amarillo in the whirlpool after the temperature has dropped below 180F (82C), and let them steep for a solid 15 minutes to really boost the hops aromatics. Chill, and ferment at 68F (20C).
I've never had diacetyl present in this beer, but to be on the safe side it can't hurt to ramp up temperatures at the end by a degree or two, and be sure to give the yeast a few days after reaching terminal gravity to clean up after themselves. It won't take long for them to chew down the available sugars, so we're still talking about a 10-12 day fermentation!
Cold crash, package, and carbonate to about 2.5 volumes of CO2, and you're good to go.
Drink this beer fresh, but don't feel the need to rush it. If you're storing it cold, it should have enough staying power in terms of flavors and aromas to be good for a good 6-8 weeks. Brew it today, and it'll be ready for your Fourth of July celebrations!