A recent article, entitled A Life of "Maybe" With Fibromyalgia & Chronic Fatigue Syndrome notes that fibromyalgia patients live in a world of "maybes." Maybe you will feel all right on a given day, but maybe not. The "maybes" make it hard to maintain a job, a social life, relationships, you name it.
Living in a world of "maybes" is a good description of daily life with fibromyalgia.
In terms of a claim for Social Security disability benefits, Social Security Ruling 96-8p has these claimants covered. According to the Ruling, a person must be able to work on a regular and continuing basis, which means 8 hours a day, for 5 days a week, or an equivalent work schedule.
If a claimant is unable to work on a regular and continuing basis, a finding of disability is appropriate. In my experience, many claims are granted for those with fibromyalgia on this basis, since there are simply too many "bad days" each month to sustain employment.