ศาลาชุมนุม

On Saturday I bought some coffee beans. I asked the seller when they had been roasted and he replied วันจันทร์ผ่านมา, which translates as last Monday. However, I'm not sure if that means the Monday of the current week (i.e. 5 days ago) or the Monday of the previous week (i.e. twelve days ago). Any ideas?

I would say it refers to the Monday of that week rather than the week before.

I tend to use แล้ว for 'last' so I would say: วันจันทร์ที่แล้ว but ผ่านมา can be used.
I use ผ่านมา when talking about a consecutive number of days. eg: ห้าวันที่ผ่านมา - in the last 5 days.... In this case แล้ว can't interchange with ผ่านมา

baker wrote:On Saturday I bought some coffee beans. I asked the seller when they had been roasted and he replied วันจันทร์ผ่านมา, which translates as last Monday. However, I'm not sure if that means the Monday of the current week (i.e. 5 days ago) or the Monday of the previous week (i.e. twelve days ago). Any ideas?

I would say that the seller's sentence can be:
วันจันทร์ที่ผ่านมา
<=
วันจันทร์ + ที่ผ่านมา

=>
วันจันทร์ = Monday
1.
ที่ผ่านมา = this -- this Monday (Monday of this week)

2.
ที่ผ่านมา = recent/last/preceding -- the action took place only a short time ago