If there is a good reason for their irregular method, then you are stuck with it until you can find another job. If there is no good reason, just custom for this method of scheduling their employees, then do talk to them about it. Just tell them it is very unsettling to not know from one day to the next if you will have to come in or not, so could they please put you on some kind of a schedule.
I can see that if the work load varies from one day to the next, they won't want to have people there on slow days, so they won't have to pay the employees for those days. But it may make more sense, in that case to have shorter work days across the board to spread the work out more evenly, so you can all have a more stable work schedule. Just in case the work should start to pile up towards the end of the week, the bosses could have a policy that Fridays, and maybe Thursdays you would all have to be available to work a full day, to keep too much work from being held over the weekend.
Hope things work out for you. 
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If a man does not keep pace with his companions, perhaps it is because he hears a different drummer.
Let him step to the music which he hears, however measured, or far away.--Henry David Thoreau