My parents were married for 50 years and, in August, my mom will be experiencing her first anniversary without her husband. For people who’ve experienced this first hand - what did people do, or what do you wish they did?
Do I get a card/gift/flowers or something?
I was living pretty far away when my dad died. My folks and a couple they were close friends with had very close anniversary dates, so the 4 of them used to go to a particular restaurant they all loved. After Dad died, I seem to recall my mom went to that restaurant with her friends all the same - in fact, she continued to do it till the friends died.
Did your mom and dad have a similar ritual that you could continue with her?
FairyChatMom’s idea sounds good. All I would add is don’t just ignore it.
Without saying directly why (in case that would be painful for her) call and say “Is there something we can do together on May 12th?”. That gives her all kinds of room to respond from “why ever on May 12th?” to “it would be nice for me if we got together and looked at pictures of your dad and shared good memories of him. Can you bring our favorite takeout?.
They did have friends with whom they would jointly celebrate their wedding anniversaries but they hadn’t done that for a number of years because my dad was sick with Parkinsons for a long time.
I live with her so it’s easy for me to do something for/with her. But suggesting Chinese takeout would be a good idea and something she enjoys.
I guess also I need to set the stage for the next few years. I don’t want to feel obligated to do a big thing every year. It’s not my wedding anniversary after all. Getting Chinese food seems like a good way to acknowledge an important day for her without going overboard. Plus I also get to eat the good food! 
You got it! Not only do you get to have a good meal 
but your mom gets to see in action that she is still loved. Win-win.
And you are a kind lad. She is a woman rich in son.
This.
After an initial couple of weeks people stopped calling, stopped remembering me for holidays. You don’t have to make a big deal, but talking to her and acknowledging the day in some manner would be wonderful.