At the same time, voters across Switzerland are being asked to cast their ballots on initiatives aimed at boosting local farming and promoting "ethical standards" in food production, including turning a moratorium on genetically modified crops into a full-out ban.
St. Gallen is expected to follow the example of the southern canton of Ticino, where a law was introduced two years ago that appeared to be aimed at burqas and other Muslim veils.
St. Gallen lawmakers late last year adopted a text which stipulated that "any person who renders themselves unrecognisable by covering their face in a public space, and thus endangers public security or social and religious peace will be fined."
The law passed the regional parliament with support from the populist right and centre parties.
Now the issue goes up for public approval Sunday after the Green Party and Young Socialists demanded a referendum.
Switzerland's government last year opposed an initiative aimed at creating a nationwide burqa ban, saying it should be up to the regions to determine if such measures are appropriate.
Voters across Switzerland are however expected to be called to vote on the issue next year after the populist right-wing Swiss People's Party gathered the 100,000 signatures needed to put any subject to a referendum as part of Switzerland's famous direct democratic system.
At the national level, the Swiss will vote Sunday on two schemes linked to agriculture and food security, urging a shift towards more ethical and environmentally friendly food production, as well as protection for Swiss farmers against cheap food imports.
The "Fair Food" and "Food Sovereignty" initiatives appear set to fail and are opposed by the government, which warns they could send prices skyrocketing and might violate Switzerland's international trade obligations.
The "Food Sovereignty" initiative, which has the backing of Switzerland's powerful farmers' union, calls for a range of measures, including turning a moratorium on genetically modified organisms (GMO) into a total ban.
Polls indicate people widely oppose GMO use in the country, but despite early signs of support, the initiative looks doomed to fail, according to a survey published this month by the Tamedia group.
Observers put the initiative's shrinking popularity down to another element baked into the text -- the call for imports to be limited to food produced under the same social and environmental norms as those applicable in Switzerland.
Opponents of the initiative warn this could cause price hikes and limit consumer choice.
Voting stations open at different times across Switzerland, but will all close by noon.
Most Swiss voters meanwhile cast their ballots by post in the weeks leading up to referenda, and near-final results usually land within a few hours of the closing of the polls.