I don't think this is possible, too little benefit to users and too much loss for PM.
User: 1 free day/year is like saving $1~1.5/year.
PM: I don't know how many users they have now, but lets assume they have 100K users. Each user get a free day per year means PM is giving away ~274 years of free service per year, which would cost them to loss more than $130K per year.
So I think this feature won't attract enough customers to cover PM's lost. I agree that this loss is relatively small comparing with PM's total gain, but there is no point to spend money on things that not worth it.
Assuming the average user stays with Public Mobile year round, then the loss of revenue would be in the ballpark of 1/365th or about one quarter of a percent. I think they could probably more than make up for that if they even grow their subscribership by a percentage point each year.