Can I wire a digital thermosat to a day and night furnace to run the fan only in the summer time?

Yes, nearly all thermostats have a small slide switch on the side that says fan ON/AUTO, if you have that then set to fan ON. If the stat is fully digital tell us the brand and model

Depends on if you have enough stat wires from furnace to stat. There should be a g terminal in the stat, and a g terminal on the furnace control board. If there is a wire hooked between these leads, when you switch the stat to fan on, instead of fan auto, the fan will run constantly.