A semi-difficult themeless … a few solving notes below, but you may want to solve the puzzle before reading them.
16A: The reference is to James Russell Lowell’s poem, “The Vision of Sir Launfal”: “And what is so rare as a day in June/Then, if ever, come perfect days ….” I’m not sure how I know it, but I believe it’s familiar enough to be fair fill.
18A: In addition to being continents, Asia and Europe are rock bands formed around 1980 and, apparently, still active. Asia’s biggest hit was Heat of the Moment; Europe’s was The Final Countdown, which was used a few years ago in a clever GEICO commercial. After a less than thorough (roughly 30 seconds) Internet search, I am fairly confident the two bands never played together.
27A: Spammers, industrial spies, data thieves and others silently hijack a group of computers (the botnet) and use them to spread spam, disseminate malware, and steal trade secrets and identities. It’s a shockingly common occurrence, even if it’s not a widely known word. To learn a lot more (and probably lose some sleep), check out Nicole Perlroth’s new book, This Is How They Tell Me the World Ends: The Cyberweapons Arms Race. It’s tense, chilling, clear, and compelling.
44A: The answer stands for Neural Processing Unit, which I didn’t know and you didn’t know (I assume). It’s terrible fill, but it’s here because my son Adam, puzzler extraordinaire and my go-to puzzle beta tester, reworked this little portion of the grid for me to eliminate three somewhat obscure answers (SCIS was SCIF (a secure compartmented information facility), STOMP was STOMA (a leaf pore), and SPUR was FARR (the actor who played Klinger in M*A*S*H). Alas, NPR became NPU, but the tradeoff was worth it.
55D: STEAM is an acronym for Science, Technology, Engineering, Arts and Math.
57D: SATB is an acronym for Soprano, Alto, Tenor, Bass, the four main choral voices.