'Normally' doesn't enter into it
IF your regular work pattern now denotes your working Sunday is on Christmas day, then you can be made to have it as a holiday day but not a lieu day unless your contract states that
What are they saying about people who are due to work on the 26th and 27th?
You might find these links useful
http://www.worksmart....ghts/Christmas_issues
http://www.direct.gov...andholidays/index.htm
http://www.adviceguid...ic_rights_at_work.htm
"Large shops (over 280 square metres) in England and Wales are not allowed to open on Christmas Day. This is regardless of which day of the week it falls on. This means that if you work in one of these shops, you must be given Christmas Day off. However, whether or not you will be paid will depend on your contract of employment."
Are you in England?