There's a danger that a spelling or grammatical error will become so commonplace that it becomes the accepted usage, and those of us trying to maintain standards are given membership to the pedants' society. Would you look twice at the following?
Photo from Jek in the box
If you have any doubt about what the correct usage is have a read of the difference between 'every day' and 'everyday'.
To check whether this problem is occurring throughout the english-speaking world I plugged "open everyday of the year" versus "open every day of the year" into my Dialecticon. 9% of usage in Australia is wrong, in the US, Canada and the UK less so. However, loosening the phrase to "open everyday", which is almost always incorrect, found the rates of faulty usage rise alarmingly to 30% in the US and 33% in Canada. At this level it could easily become the accepted norm.