Both mean the same thing. The second one seems to add “do” to reinforce the idea that good things happen on Fridays, but it's not a necessary addition.
The statements themselves don’t say anything instructive without some context. They only confirm that good things have been known to happen on Fridays. They say nothing of whether bad things may also happen on that day nor whether good or bad things might happen on any other day.
The intent of the phrases would likely be to mean that Friday is generally considered a favorable day. In many western cultures, at least, Friday often marks the last working day of a week. So Friday is often viewed as a happier day than much of the week because it begins transitioning into the weekend, where your time is for your own wants and needs and not for your occupation. In this meaning, saying good things happen on Fridays is indicating that Fridays tend to be positive and days where you are more likely to enjoy them more than other days, especially other work days.