I my experience the memorial candle is lighted early before the first guests arrive. Often the coordinator or another designated person lights it. The placement of the candle will depend on the church and how much space is available and where. The candle is often placed on a small table at the front to the side of the area where the ceremony will occur. I have also seen it placed in the entry way if it is fairly large, often with a picture of the deceased.

I've always thought the remembrance candle was lit DURING the service, up front, in a little pause/ceremony. Just after or just before the vows, I don't remember. But the officiant makes a 2-sentence introduction, and pretty music is played, and the bride and groom go together to light the candle.

of course, the only weddings i go to anymore are Catholic ones, and Catholics have the "light a candle as part of a prayer" tradition or rite. And so there's this quiet moment of prayer during the lighting--prayer of thanks for that person's life, prayer for comfort for those who miss him/her, and even prayer TO that person that they intercede before the throne of God on bride & groom's behalf.

(an interesting part of the Catholic faith--shared by us Lutherans, I have found out--is that people who've died are still alive in heaven, and can add their prayers on our behalf to our own)

Your son and his bride might ask their officiant what he/she thinks.

If you light it before the ceremony begins or before the guests arrive, then those people don't get to participate in that moment of collective remembering. But of course, you can have your statement of sentiment steadily communicated.

I don't think there's any hard-and-fast rule; just whatever they like.

Well now that is all over I can tell you what we ended up doing. Off to the right of the alter on the floor close to in front of the pews we had a table. On the table were pictures of their grandparents that had passed on. Also there were 2 remembrance candles (1 for each side of he families) then the 2 separate tapers for Moms to light and then the couple to light from to light the Unity candle. The Unity candle was in the middle of the table. The Remembrance candles were burning already when the cermony started. I, as mother of the groom lit the single taper for our side From the remembrance candle. then the Bride's mom lit their taper from the Remembrance candle for their family. During the ceremony the couple lit the Unity candle from the 2 tapers. It was quite moving to see it all come full circle and to be a part of that circle.