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If you're a freelancer, regularly deciding you don't fancy working odd days is not conducive to people wanting to employ you for regular work.
Also most freelancers will have a contact which sets out what they are required to do in order to get paid. If as a freelancer you're required to work 4 days a week on one job, then you work 4 days a week on that job. Unless you negotiate a change or exception to that contact, you can't just announce you won't be there on that day. If I (I freelance) am booked to do 5 days on an event in London. I can't decide a couple of weeks before the actual job that I'd rather not do the Sunday, because I may well find my employer decided to cancel my entire job. It's also going to mean I'm less likely to be booked for the next job they do.
Also most freelancers will have a contact which sets out what they are required to do in order to get paid. If as a freelancer you're required to work 4 days a week on one job, then you work 4 days a week on that job. Unless you negotiate a change or exception to that contact, you can't just announce you won't be there on that day. If I (I freelance) am booked to do 5 days on an event in London. I can't decide a couple of weeks before the actual job that I'd rather not do the Sunday, because I may well find my employer decided to cancel my entire job. It's also going to mean I'm less likely to be booked for the next job they do.