I just randomly thought of this while packing today..will there be many formal events at Mac? Should I bother bringing a pair of heels and a formal dress with me or just leave those at home?

There's definitely a residence-wide formal every year, but that's typically in 2nd semester, so you may be able to wait until then to bring your fancy clothes. Your building and/or faculty may also have its own formal event later in the year (e.g., the Humanities wine & cheese), but overall, other than that you probably won't have much use for formal attire.