Not to mention the accuracy, I think a weekend means some weekend whereas the weekend mean a particular weekend you had intention to talk about. Also, I think over the weekend may exist but its meaning may vary depending on what precedes it. Hope this help.
“Happy hour” is a time after the work day is over, when bars serve alcoholic drinks at reduced prices. Often the bars have it from 5:00 p.m. to 6:00 p.m. or later on Fridays, but some of them have it on other days.