How to find this price of the specific candle?

[BlueMoon]

I am finding this hard to figure out myself so posting it here.

I have a bucket of trade. I want the open price of the weekly candle which is before the oldest trade in this bucket of trades.

What I mean:
I could have several trades in a bucket that I can easily define. Lets find out which trade is the oldest trade in this bucket. Once we find out the Date/Time of this trade...we can easily know which week this trade was taken. I want the open price of the weekly candle before this.

Can someone please help me with this?