Cornus Florida, the regular white flowering dogwood, is hardy in zones 5-9. If you purchase a large enough specimen it will have some flowers in the spring. Fall planting is a plus so that the tree can get established without supplemental watering. Most big box stores only carry them in the spring but a nursery should have some available.

Zone 6 covers a lot of territory....where specifically? There seems to be some disagreement about the viability of fall planting in some areas of the country although if hardy for your zone, you should theoretically be able to plant successfully as long as the soil remains workable. Personally, I wouldn't think twice about fall planting. It is typically advised over other times.

It depends on the size and maturity of the tree in question, but dogwoods will typically bloom even in a nursery setting. Seed grown trees often do take a few years before they flower.

It will be around 5-6 feet and I’m in Massachusetts

Fall planting, before Thanksgiving, will be fine. Just be sure to water it if you have a dry fall and winter. Just because it's naked doesn't mean it doesn't get thirsty.

When it fully blooms depends on what kind of white dogwood you got. Is it a seed grown species, grafted cultivar, a Kousa dogwood? In general at five to six feet, you are three springs away from the start of good bloom - site conditions depending.

USDA 6 is the coldest zone where routine fall planting is feasible. Below that it needs to be checked that kinds being installed are extra hardy, unlikely to be bothered by severe cold coming before establishment. Because anything planted late enough in fall for the timing of this to be after winter stem buds have been set will not make significant root growth before spring bud break. Also any roots that are cut during the fall planting process will not grow again until spring.

Fall:

Mature winter stem buds send hormones to root tips that cause existing roots to elongate markedly. As in 60% of the annual total increase in root length

Winter:

Little or no root activity as cold climate adapted plants basically hibernate

Spring:

Opening winter stem buds send hormones to the root system that causes new roots to be produced

This is why roots of stock dug and bare rooted in November and placed in cold storage until late winter- early spring shipping times show no signs of fresh growth until bud break.