One trait of successful user groups is don't play calendar roulette. Meaning: they chose a time or day each month to hold the group meeting.

We meet every 2nd Tuesday.

or

We are the last Thursday of the month.

Why is this important?
You want your members to be able to determine meeting dates based off a quick glance of a calendar.
Consistency is key. If your meetings are in different venues or different days of the month, you're chancing that someone will forget and not be in attendance.

What's a good night for meetings?
With the Hampton Roads .NET Users Group, I chose the 2nd Tuesday of the month. Why? Because I felt Tuesdays were a better for my schedule. Only once in a 5 year period did I change the meeting.

My recommendation is to choose a Tuesday, Wednesday, or Thursday. Mondays are bad for folks because it's the first day back to work after a short weekend. Fridays are bad because, dude, it's the weekend. PARTY TIME.

Be careful with Wednesdays as well. Many church functions occur on Wednesday nights, and that might be limiting a percentage of your population.

When is it okay to move the meeting?
Let's imagine you have a speaker coming into town that's "a big deal". But they're only able to meeting on Wednesday night, and your meetings are Tuesdays. MOVE THE MEETING. Make sure the change is well communicated, and that it's temporary. You might lose attendance that month, but people are adaptable. Just don't make it a habit.

Conclusion
Keep scheduling simple. Hampton Roads .NET members knew where to go and when to be there every month of the year. If you build a level of consistency, you're going to have people showing up out of habit, and that's a recipe for a successful group.