Why do you say have blessed day instead of have a bless day?

Answer 1

The original term, "May God bless you" was contracted into "God bless you," or sometimes just "God bless."

The verb "bless" is used in this phrase instead of "blesses" because it is in the subjunctive mood. The subjunctive mood expresses a hope, wish, possibility, or opinion. The subjunctive mood is hard to distinguish in modern English because it often has the same form as the indicative mood, which indicates a factual statement. The subjunctive mood is easiest to see in the 3rd person (he, she, it) or in this case "God."

NOTE: Most modern-English speakers don't use the subjunctive mood regularly, but it lingers in phrases that have been around for a long time, such as "God bless you" or "Long live the queen." "Long live the queen" doesn't express a fact that the queen spoken of has lived or is living a long life, (which would be expressed by saying "Long lives the Queen") but rather the hope of the speaker that the queen will indeed live a long life.

SUBSEQUENT ADDITION: The subjunctive mood in such constructions has virtually disappeared from British English but is very common in American English, where it is standard. American: "They demand that the fighting come to an end" (present); "They demanded that the fighting come to an end end' (past). British: "They demand that the fighting comes to an end" (present); "They demanded that the fighting came to an end" (past). The American usage requires the subjunctive mood regardless of the time of the action, the British the simple present and simple past (and future when appropriate). There are British grammarians who regard the American usage as more faithful to historical English usage and the British usage as a regrettable modernism.

Answer 2

Grammatically, "bless" is a verb ("to bless"). The priest will bless the congregation. But only an adjective can modify a noun, so you have to change the word into an adjective-- blessed (pronounced as two syllables-- BLESS- ed). Pronouncing it differently helps to avoid confusion with the past tense of the verb "to bless," which is only one syllable (rhymes with dressed, or stressed). Thus, as a verb: The priest blessed the congregation. And as an adjective, describing the noun "day": I wanted you to have a blessed day (pronounced with two syllables, to show it's an adjective here).

Answer 3

Because it is expressing a wish or a hope, and not describing something God is doing. You are hoping that God will do it; in fact, you are asking Him to do it. (May) God bless you. In other words, I hope that God will bless you. This expression comes from hundreds of years ago when people were very superstitious and they believed demons and evil spirits were everywhere. They asked for a blessing from God, because they believed this would give protection. (A sneeze was considered by some cultures to be a sign that there was an evil spirit; the name of God was supposed to drive that spirit away.)