This is an extremely important day. It may mean the anniversary of your child’s passing or their due date. For me it is the day my Milo took his last breath. He took it in my arms while his dad’s surrounded me. He was just under 5 months old.
It needs to be marked in any way you feel fit. If you need to be by yourself then do it. Involve your living children if you want too or don’t but make sure the day is marked. Your child only has a few special days.
The anniversary is a day where is it sociably acceptable to cry in public or even to just scream their name.
My Milo’s anniversary is in early July. This year will be the fifth one and each year it gets harder and harder, especially as his siblings get older and older. In previous years we have done balloon releases (don’t do them now as I am more aware of the environmental repercussions), read him books sitting by his grave, had family picnics in the space behind his headstone (got some funny looks that day) and visited a beach to write his name in the sand. I have no idea what to do this year. I would like to do something that involves his big brother and little sister but both of them get very bored, very easily.
A Milo shaped hole. 
I am sorry for your loss. A friend just marked the second anniversary of her son’s death and talked about how hard it was leading up to and the actual day. May you find comfort as a family. I think a picnic at his grave is a great way to mark the day.