Copenhagen in May gives you long daylight hours, pleasant weather for walking the canals, and fewer summer tourists crowding the design shops. It is the best time to see the city at a comfortable pace without rushing through the streets.
A typical trip runs about $2900, which is a lot to hand over at once. Spreading that out over time makes it easier to manage if you plan the payments carefully.
The intro window is a deadline, not free money. If you still have a balance left when the 15 months are up, interest kicks in on what you owe.