Middle Child’s BLT season starts tomorrow. This is how they prepare for the frenzy.

Philadelphians know it’s officially peak summer thanks to a recent announcement: the beginning of BLT season at Middle Child, where the seasonal sandwiches draw lines down the block.

“We become Pat’s and Geno’s but instead of making cheesesteaks all day, we’re making BLTs,” said Matt Cahn, the owner of the Center City sandwich shop Middle Child and Fishtown’s Middle Child Clubhouse. You won’t see fresh tomatoes on the menu at any other time of the year, but come mid-July, “It’s just BLT, BLT, BLT,” said Cahn.

You could say tomato season is Middle Child’s Super Bowl. It is intense but short. BLTs are only made for about four weeks, when tomatoes are at their best. It starts tomorrow at 11 a.m. (9 a.m. on ensuing days) until sell-out. Each location makes roughly 200 to 250 sandwiches per location per day.

While there are other top-tier tomato sandwiches in town — don’t miss the River Twice entry, adorned with gamtae and a quenelle of caviar, for example, or Loretta’s in Headhouse, with herbed aioli and the same vaunted produce as Middle Child’s — none stir up quite as much fervor.

Middle Child’s tomatoes, cut into thick, rainbow-hued slabs, always come from Urban Roots Farm in Newtown Square. Tomatoes arrive at peak ripeness, and they only have a shelf life of about two days.

“These tomatoes are really expensive,” said Cahn. They’re about $38 to $40 per flat or case, which means there’s $2.60 worth of tomatoes on each sandwich. “It costs more than the turkey does on turkey sandwiches.”

Their classic BLT ($16) consists of those tomatoes, five pieces of Leidy’s bacon (“Nothing fancy ... because the sandwich is all about the tomatoes,” said Cahn), Duke’s mayonnaise, and locally grown arugula dressed with tomato vinaigrette all sandwiched on slices of rye bread from Merzbacher’s bakery. The sandwich can also be made vegetarian (substituting bacon for avocado), vegan (with avocado and veganaise), and gluten-free.

The BBLT ($21) is served on a LeBus ciabatta, and that second “B” stands for a whole ball of Jersey-made burrata from Lioni Latticini. It also features a chili-tomato jam, made of tomato scraps and usually Calabrian chilis, though Cahn has been experimenting with red chili flakes and gochujang this year.

For the first time this year, Middle Child Clubhouse will offer a slightly different evening BLT, with thick-cut slab bacon, for $18 — if Clubhouse’s tomato supply lasts till dinner service. Canh said he expects even more tomatoes in a week or two, meaning more availability for dinner BLTs.

This is Middle Child’s ninth tomato season since opening in 2017, and every year, the cult obsession with the sandwiches has grown. Every season, Cahn tweaks the processes of both selling and making the sandwiches.

Tomatoes are particularly delicate. They’re perfectly ripe for only a day or two before they start turning to mush and growing mold. And because they’re cored and sliced to lie flat, they generate a lot of waste. The tomato scraps, along with tomato “seconds” — or tomatoes a little too soft to slice — go into that chili-tomato jam and the tomato vinaigrette that dresses the arugula.

The base for both of those sauces is tomato water, which Middle Child’s team collects every day.

“Every morning we treat the tomatoes like steaks. We season them a couple hours before service with Diamond Crystal kosher salt, then shingle them on a perforated pan and layer the shingles,” said Cahn. Gravity presses down the layers and the tomato water accumulates below. It’s a boon to Middle Child Clubhouse’s bartenders: In past years, they have made tomato agua fresca, tomato water ice, frozen tomato pops, and tomato martinis with the salty tomato water.

There are some loosely enforced rules during BLT season. First, you should eat the sandwich within 10 minutes of receiving it.

Second, embrace the wait. Middle Child will turn off online orders in the thick of BLT season to prevent customers from jumping the actual line. Cahn encourages customers to enjoy the madness. “Part of the fun is going and [waiting]. People are chatting and getting to know each other and there’s a lot of debate over whether or not it’s worth the hype,” said Cahn.

“The first week is really crazy, but in a couple weeks, once the tomato plethora has really come in,” the lines get more manageable, Cahn said. And they move quickly, thanks to a full week of BLT training, in which the staff practices until they’ve mastered every detail, down to meticulously wrapping the final product.

“The sandwich is a structural game of Jenga,” said Cahn.