John Hannett was addressing delegates at the Labour Party conference in Brighton amid Conservative proposals to devolve Sunday trading laws to local authorities, which could mean shops can be open longer on that day of the week.
The Labour leader jeremy Corbyn backed opposition to extending Sunday trading laws, leading to praise from some Christians.
Mr Hannett acknowledged that Sundays are important for many people because of family and religious activities like church.
He said: "Sundays are so precious to our members and they face losing the few hours they can spend with their family.
"Over ten thousand Usdaw members told us that Sundays are too precious to be boring, like ministers claim. Sundays are about watching your child play football, visiting an elderly relative, getting together for a family meal or many other community, religious and family activities.
"Sundays are special for most of us, not just shopworkers. However, if shops open all day more and more people will need to work. Delivery drivers, warehouse workers, cleaners, bus drivers, the list goes on. Before long Sunday will be just another day like any other.
"The Sunday Trading Act is a great British compromise, which has worked well for over 20 years and gives everyone a little bit of what they want. Retailers can trade, customers can shop, staff can work; whilst Sunday remains a special day, different to other days, and shopworkers can spend some time with their family."
Usdaw, the shopworker's union, claims extending Sunday trading hours will affect three million shopworkers across Britain by forcing small businesses to shut, making people unemployed and stopping employees spending time resting or with loved ones.
Communities minister Brandon Lewis has previously said: "This Government is determined to devolve powers previously held in Whitehall to local people.
"We have already taken a range of measures to boost the Great British high street and now we are giving local areas another tool to encourage shoppers to the town centre and get shops to grow and thrive."
