Back when I was on my last maternity leave, forever ago, I joined a local Mom's playgroup.
At the time, it was sort of fun. We met up on the long boring weekdays and chatted. (We never ate, for some reason. We'd have these playgroups going from 10AM to 2PM and god forbid anyone would ever suggest having lunch. Mothers don't consume food, right?)
Eventually, I went back to work. And that was it for the playgroup and my playgroup friends.
I still get emails about the playgroups. They're always on weekdays, and even if I happen to have the day off, I'm usually playing catch-up on errands. And whenever I suggest doing something on a weekend, everyone tells me that's family time.
I think this is why working moms and stay-at-home moms can't really be friends, not because we have nothing in common. SAHMs want to fill their weekdays with activities, whereas on weekdays, working moms are either working or too exhausted to contemplate going out.
Still, it makes me sad every time I see those playgroup emails.