For some reason there doesn't seem to be any official reminder sent out when it's time to submit a VAT return, not from my accounts software or HMRC.
I thought it would be easy to just add a reminder for the 2nd Friday every 3 months but my Google Calendar didn't seem to support the "2nd Friday" bit, so I ended up creating an ical file which contains the appropriate RRULE and DTSTART and then importing it. Google Calendar shows the events correctly on the second Friday every 3 months, but trying to edit the event does show an "unsupported recurrence" message.
The file can be downloaded here: vatreturn.ics.
And looks like:

BEGIN:VCALENDAR
PRODID:-//no.org/no.app//EN
VERSION:2.0
BEGIN:VEVENT
SUMMARY:VAT Return
TZID:Europe/London
DTSTART:20240209T160000
DTEND:20240209T170000
DTSTAMP:20240117T133013Z
UID:4e936cfb-ed93-4ae0-9790-6b8bc94bf04c
RRULE:FREQ=MONTHLY;BYSETPOS=2;BYDAY=FR;INTERVAL=3
END:VEVENT
END:VCALENDAR

Not really a Linux thing, but I did use vim on Linux to edit the ics file :)