It’s the day the college basketball world has been waiting a year for, their national holiday. Selection Sunday has arrived and plenty is still undetermined.

Before the brackets are set, The Post’s Zach Braziller breaks down the much-anticipated selection show:

The biggest question is the fourth No. 1, Purdue or UCLA.

The Boilermakers have the slight edge due to more Quad 1 wins, 10 compared to eight with one fewer loss, a better record away from home (14-3 to 12-5) while coming out of a far better league.

The Big Ten rises well above the Pac-12. I also think the committee will be wary of giving UCLA the No. 1 seed after it recently lost starting guard Jaylen Clark to a torn Achilles.

Alabama and prized freshman Brandon Miller should get the top overall seed courtesy of their 18-5 Quad 1 and 2 record, ninth-ranked strength of schedule and dominance of the SEC.

Kansas, with a mind-boggling 17 Quad 1 wins in 24 opportunities, gets the second No. 1 seed followed by two-loss Houston.

The Cougars can’t compare with the other three when it comes to the overall level of competition, but their No. 1 NET ranking, elite efficiency numbers (the only team ranked in the top 11 on both ends of the floor) and 22 wins by double figures make up for that.

One of the major questions surrounds Vanderbilt, which has performed like a top-25 team over the last month, winning 10 of its last 12 games and beating projected at-large teams Kentucky (twice), Auburn and Tennessee.

The Commodores’ NET ranking of 79, however, would be the second-lowest to receive an at-large bid, after Rutgers got in at No. 80 last March, and their three sub-Quad 2 losses are a detriment.

Arizona State’s late-season wins at projected two-seed Arizona and over USC in the Pac-12 Tournament may have gotten Bobby Hurley’s team in the field.

Nine Quad 1 and 2 wins, and just one bad loss, work strongly in the Sun Devils’ favor.

Two high-majors from the Northeast, Providence and Pittsburgh, seemed safe before shaky finishes.

Providence has lost six of 10, and has just three wins over expected at-large teams, which puts it in a precarious position, despite a quality NET of 56.

The three expected at-large teams the Friars beat, it should be noted, are Connecticut, Marquette and Creighton, high-caliber opponents all in the top 16 of the NET.

Pittsburgh’s résumé also looks good enough at first glance — seven Quad 1 and 2 wins, only two bad losses and a 67 NET — but only two wins over at-large teams is concerning.

As important as metrics are, beating tournament teams is something the committee emphasizes annually.

It’s why North Carolina State and Oklahoma State, despite both boasting strong NET rankings, 43 and 45 respectively, are right on the cutline.

Both have poor records against the at-large field, N.C. State is 2-5 and Oklahoma State is 4-12.

This entire group was able to exhale when UAB lost in the Conference-USA title game to tournament-lock Florida Atlantic, meaning there was no last-second bid-stealer.

Rutgers won’t be able to relax until it sees its name flash across the television screen (or however the team follows the selection show), but the Scarlet Knights may have punched their ticket with their Big Ten Tournament opening-round victory over Michigan.

A second straight trip to Dayton for the First Four is possible. The Scarlet Knights’ résumé is wonky: 10 Quad 1 and 2 wins, but four Quad 3 losses.

Their NET ranking of 40 is helpful, and so are their quality road wins over projected tournament teams Purdue, Northwestern and Penn State. A 342nd-ranked non-conference schedule, however, is not beneficial.

The Scarlet Knights will have Garden State company from Fairleigh Dickinson, the automatic qualifier from the low-major Northeast Conference, which will likely have to play its way into the main draw from Dayton.

After FDU won just four games a year ago, the Knights, under first-year coach Tobin Anderson, will be thrilled to be part of the festivities.

After winning the MAAC Tournament for the sixth time in eight years, Iona is headed back to the dance, probably as a 13-seed.

That’s higher than usual for the conference, a credit to Rick Pitino challenging the Gaels with the 59th-ranked non-conference schedule.

Meanwhile, Fordham’s hopes of reaching the tournament were ended by Dayton on Saturday afternoon in the Atlantic 10 Tournament semifinals.

The mid-major conference, suffering through a down season, is a one-bid league this year.