Update: Nintendo of Europe has confirmed that the Booster Course Pass Wave 1 content will go live at midnight CET, which is 11pm UK. This matches up with the Australian release time, but there's no confirmation yet that it'll arrive in NA at its equivalent times - 4pm Pacific / 7pm Eastern. [The new tracks seemed to go live at midnight local time in Japan]
Original Article: We're not sure why, but Nintendo has an odd insistence on not confirming release times for digital versions of games and DLC. On most occasions games land at midnight local time, but sometimes they don't, so we have to resort to checking our Switch late at night and crossing our weary fingers.
That's been the case with the Mario Kart 8 Deluxe Booster Course Pass, with Wave 1 landing on 18th March at some point. GoNintendo has spotted an actual confirmed time via Reddit, however, which was found on the Australian product page.
According to that page it'll arrive at 10am AEDT on 18th March, making it a fun morning down under. In terms of how that time converts to other territories, that's midnight CET on 18th March, which is 4pm Pacific / 7pm Eastern / 11pm UK today (17th March).
The fact it's evidently not a midnight launch everywhere (in which Australia would have its usual headstart) and that MK8 Deluxe has server maintenance today (at 1pm UK time for an hour), does suggest these could be the roll-out times. That said, the Nintendo of America store page still says it's out on 18th March, so it could go either way.
If you're in Australia, though, now you know!