pre-Christmas period is the busiest time of year for shopping in France; many shops extend their opening hours, notably by staying open at lunchtime, and - during the last three weekends before Christmas, opening on Sunday afternoon. Sunday opening is particularly common in towns and cities that have Christmas markets. Generally speaking, all shops that do a brisk trade in the runup to Christmas stay open until at least 7 p.m.
Out of town supermarkets and shopping centres tend to remain open slightly longer in the period before Christmas, and in particular are open on Sundays (typically from 10 a.m. to 7 p.m - but it is always important to check locally).