We chose our anniversary to be March 13th of 2018. As to how we got to this date, well, I know saying this here will just re-confirm the whole "le reddit user" perception, but I did just take the first post I made in relation to her/waifuism in general as the date.
The reasons as to why I chose that as the date are a little more specific though. As I watched through Re:Zero, I primarily felt for Emilia, this was ultimately re-affirmed when I started going into reading the source material (the web novel, not so much the light novel) and increasingly felt for Emilia as I was reading and eventually did just post a thread on Reddit about whether or not this was love. I questioned it, not really because I had doubts over a waifuist relationship, but I didn't know if I could feel such over a character, especially when I had already been pretty apathetic in regards to relationships and so going from near no relationship to one with a fictional character honestly seemed like more of a step to me than going from no relationship to one with a real person. As for that, there's reasons why I thought so but I can answer that elsewhere. As such, I chose that date as the start of our relationship.