Alright, now what's the difference between μέρα and ημέρα? Is it like οκτό/οχτό, επτά/εφτά, αδελφός/αδερφός? Does it have to do with katharevousa and demotic?
Another alternative form that bugs me for some reason: εννέα and εννιά. In Greek school as a child (long long ago!) I learned nine as εννέα but the Pimsleur course uses εννιά. I just looked it up in Wiktionary and it actually gives a statistic on usage with εννέα being used 75% of the time and εννιά only used 25% of the time. That makes me feel better about sticking with εννέα! I like knowing about the alternate forms so that if I ever do get to go to Greece I will have a better chance of understanding people.
Believe me, as a native, I hear εννιά being used way more often than εννέα, for some reason. So if you ever visit Greece, I bet εννιά is something you'll hear pretty often as well. But εννέα isn't wrong, so don't worry about it! ^.^
I bet wictionary gives usage percentages in written speech. I'd say that in written speech εννέα is more common, but in verbal it's εννιά.