So in my case, I was not prior enlisted, I commissioned in May, got out at MSR, became a reservist with no break in service in December....
....my anniversary year is based around May, right?
No, the other way around. Think of it like your reserve birthday.
For you, your pay raises still happen in May though.
Your drills an orders reset every October 1st (fiscal year), so theoretically you could screw yourself by doing all of your FY21 reserve days in Oct-Nov 2020, followed by doing nothing until the summer of 2022, but then doing all of your FY22 stuff that spring or summer. This would create a giant gap, starting on your reserve birthday in December 2020 all the way through to December 2021. You'd get a year older in the reserves and a year closer to mandatory retirement, but that giant gap would mean that you're forfeiting a
qualifying year (good year) towards retirement. A few guys out there have made this mistake a few too many times and when they got to the end of the road then they had to separate with no retirement (no pension).
It's still something to watch in the future if you have some weird timing coming back from a deployment, taking several months off from the Navy, and then heaven forbid you have a life event like a medical condition that prevents you from doing reserve stuff. Sure, there's a waiver for everything but not needing a waiver in the first place is easier.
More importantly, educate yourself on the ins and outs of this minutia so that you can help out your fellow reservists.