Many years ago I was a senior manager with British Home Stores, and for a time I was based at the Birmingham store, where about 30% of the staff were Muslim.
During that time I had absolutely no issue at all with Muslim staff taking time off for Eid al-Fitr and Eid al-Adha. The uncertainty of the date, often unknown until 24 / 48 hours beforehand, caused major staffing issues, but respecting their customs and faith was the right thing to do.
I understand why many people struggle to accept Marks and Spencer's latest decision, but we live in an increasingly multi-faith., multicultural world. If we, as a society, are sufficiently accepting and tolerant to allow same sex-couples to marry, then not insisting that a Muslim member of staff sells you a pork chop or a bottle of Jurgemeister really isn't going to bring down society, is it?
I appreciate I hold an exceptionally liberal view on these matters, which will be alien to many friends and colleagues, but I passionately believe that we earn respect, friendship and understanding by showing respect, friendship and understanding.