Members of Wales's 460 non-conformist chapels are boycotting supermarkets for the day and using small rural shops, post offices and garages instead.
Chapelgoers are worried about the plight of small businesses
They are holding a "thanksgiving day" and will only shop "locally" in support of the rural economy.
Church leaders say small businesses are under threat and the livelihood of many of their 30,000 members is at stake.
They are also inviting members of other Christian and religious denominations to join Saturday's day of action.
The campaign started in Carmarthenshire but other chapels across Wales have joined in.
The Reverend Kevin Davies said many village shops, post offices and petrol stations faced closure and "the very fabric" of rural life in the "Welsh-language heartlands" was under threat.
"We believe it is our duty as Christians to challenge the system which is allowing this to happen," he said.
Earlier this year at its annual conference the Union of Welsh Independent Churches called on the Welsh Assembly Government to give priority to the farming industry in Wales, and to do more to promote local produce.
Alun Lenny, who proposed the motion, said: "I realise that this may be seen as a symbolic action but it may divert some people from doing all their shopping in the supermarket.
"Even buying goods once a week at a local shop could make a big difference to that business.
"In the present economic climate in rural areas, fair trade needs to be applied here, as well as in poorer countries in other parts of the world."