Some things you just know without having to consult a calendar: Valentine's Day is Feb. 14, July 4 is Independence Day, and Philadelphia public school students return to class after Labor Day.

That last part is about to change.

Beginning this year, all Philadelphia School District students will report to school on Monday, Aug. 27 — a full week before the unofficial end to summer. Officials said the pre-Labor Day start is the new norm in Philadelphia, mostly to front-load the school year with as much instructional time as possible, and are in the midst of a splashy, city-wide social-media campaign to raise awareness of the change.

An analysis of school start dates around the region shows that, as usual, they're all over the map — Mastery Charter Schools, a large network in Philadelphia and Camden, starts Thursday, Aug. 23, and many districts in New Jersey don't go back until Sept. 6. That's for a number of reasons, experts say: considerations regarding snow days, holidays, families' summer vacation plans, standardized testing, professional development, and an attendance falloff late in the school year.

Though pre-September start dates are on the rise nationally, many districts locally still opt for the traditional post-Labor Day start. In Lower Merion, schools spokesperson Amy Buckman said the schedule suits families well.

"Lower Merion schools have started after Labor Day for as long as anyone seems able to remember, and there doesn't seem to be any reason to change,"  Buckman, a longtime district parent herself, said in an email. "I can say it's what our district families are used to, and they make their summer plans accordingly."

In some states, schools are prohibited by law from starting before Labor Day. (Thank or blame the amusement-park industry, experts say — in Virginia, the so-called Kings Dominion law, lobbied for by the tourism and hospitality industry, means September start dates unless schools get a waiver from the state.)

Neshaminy School District welcomes its students back on Aug. 30, though kindergartners and fifth and ninth graders start Aug. 29. Following a growing national trend, the district has started in August for the last several years, though there was some back-and-forth, said Chris Stanley, a district spokesperson.

Superintendent Joseph Jones III and his cabinet craft the calendar with many considerations in mind, Stanley said, but the two biggest are building in a cushion for snow days and ending early enough in June (the last scheduled day in Neshaminy is now June 14) to allow for the Disney trips, camps, and other summer rites that many district families cherish.

"By starting two or three days before Labor Day, we're able to give ourselves a little bit of wiggle room and get everybody out on time," said Stanley. "It's built around the needs of our community."

Mastery Charter students return to class Thursday. Students return early because Mastery builds in extra time throughout the school year for teacher planning and training, said Scott Gordon, the organization's CEO.

"We put a priority on ensuring that teachers have the training and support and development to be successful," Gordon said. "We started earlier to make sure we could accommodate all that time."

The Phillie Phanatic holds the hand of Zariah Brown, 5, who is about to begin kindergarten at Gilbert Spruance Elementary, during a news conference to promote Philadelphia’s first day of school, Aug. 27.
TIM TAI / Staff Photographer
The Phillie Phanatic holds the hand of Zariah Brown, 5, who is about to begin kindergarten at Gilbert Spruance Elementary, during a news conference to promote Philadelphia’s first day of school, Aug. 27.

In Philadelphia, Superintendent William R. Hite Jr. said the shift to a pre-Labor Day start beginning this year — which was discussed for a year prior to being decided by the old School Reform Commission in 2016 — was precipitated in part by a desire to stack the year with as many uninterrupted weeks of class as possible. With a midweek post-Labor Day start and a number of holidays early in the year, the first chunk of the term had multiple interrupted weeks, with low attendance those weeks, Hite said.

"We wanted to start with a full week, and get into a routine," said Hite. (Sort of a full week, actually. Philly students have a regular week Monday through Thursday, then a half-day on Friday, followed by a day off  Monday, Sept. 3, for Labor Day.)

Another major reason:

"We wanted more instructional days earlier in the year," Hite said. "Closer to the end of the year, you get into spring-itis and summer-itis. This gives us more days to be in front of children before they sit for any assessments — state, Advanced Placement, SAT."

Attendance typically tanks after Memorial Day, and, barring any changes for snow days, the new calendar will have Philadelphia students finished school on June 4. They previously had gone well into late June.

When the change was first proposed, there was pushback — some staff and parents were and are concerned about hot temperatures in largely un-air-conditioned buildings, but district officials said that they studied temperature trends and that they found little difference in the heat in late June and late August. (The city's teachers' union has disagreed on this point, raising concerns about staff and student welfare in large, old buildings after a summerlong buildup of heat.)

Philadelphia is thoroughly publicizing the calendar shift with a #RingtheBellPHL hashtag, selfie videos, and reminders from everyone from Phillie Rhys Hoskins to Mayor Kenney to show up for school Monday.

Superintendent William Hite speaks during a news conference promoting the School District of Philadelphia’s first pre-Labor Day start.
TIM TAI / Staff Photographer
Superintendent William Hite speaks during a news conference promoting the School District of Philadelphia’s first pre-Labor Day start.

Inertia often guides school calendars, said Kenneth Gold, an academic who studies the history of summer break.

"There have been very few moments where traditions were completely upended," said Gold, dean of the education school of the College of Staten Island/CUNY. "Very often, when they are, it engenders a lot of resistance."

In the 19th century, school calendars varied tremendously: Urban schools were typically open for nearly the whole year, and rural schools much less. (In 1842, according to Gold's research, schools in Philadelphia and surrounding towns were open an average of 251 days of the year.) But when compulsory school became the norm, rural school years got shorter and urban ones longer; a summer break was built in, in part, to professionalize the teaching force by giving teachers time to hone their craft, Gold said.

These days, while there are some regional differences — districts in the South tend to start earlier in the summer, for instance — the basic school-calendar model remains the same: roughly 180 days of instruction.

A sampling of local school start dates:

Pennsylvania

Bensalem: Monday, Aug. 27, grades 2 through 6, 8, and 10-12. (Kindergarten, first, seventh, and ninth graders start Tuesday, Aug. 28, then all students attend Wednesday, Aug. 29.)

Cheltenham: Monday, Aug. 27

Downingtown: Monday, Aug. 27

Lower Merion: Tuesday, Sept. 4

Neshaminy: Thursday, Aug. 30 (kindergarten, fifth and ninth graders return Aug. 29)

New Hope-Solebury: Tuesday, Sept. 4

Mastery Charter Schools: Thursday, Aug. 23

Philadelphia: Monday, Aug. 27

Tredyriffin/Easttown: Monday, Aug. 27

Upper Darby: Tuesday, Aug. 28

New Jersey

Camden: Thursday, Sept. 6

Cherry Hill: Wednesday, Sept. 5

Gloucester Township: Thursday, Sept. 6

Haddonfield: Wednesday, Sept. 5

Lenape Regional: Tuesday, Sept. 4

Monroe Township: Thursday, Sept. 6

Mount Laurel: Thursday, Sept. 6

Pemberton Township: Thursday, Sept. 6

Washington Township: Wednesday, Aug. 29