Most "program" codes only go 90 days out max, so April won't be available yet. The group and retailer discounts (CAA/Costco/YourCompany) are always available.
I make my best deal reservation on-line in advance, and go back when the codes come out. I currently have a reservation for an SUV (at the midsize rate) with National in March. In January I'll start looking for a discount code and add it to the reservation I already have. National allows this; YMMV with others. I also joined Emerald Lane as it speeds pickup and waves the extra driver charge for DW.