I bought 13 window boxes for the spring and im not sure what to fill them with.
If you plant now you will still get 4 months, using pansies, primroses, batchelors buttons or any other spring bedding you can find. It is too late to work in some spring bulbs as they are already beginning to flower.
In May, you can plant up with a choice of summer bedding plants. More about this at: https://gardenie.wpengine.com/gerrycategory/summer-bedding-flowers/?id=5434