how long before the event do you send out invites?

How long before the event do you send out invites?
Im having a 21st birthday party and just wondering what the standard amount of notice you give guests?
Do you send them out a month prior? More? Less?
Many of my friends like me have children and i dont want people to feel like i didnt give them enough notice to arrange a sitter ect.
Ive posted this in the pregnancy section as it seems to get the most views.
Thanks

Best Answer - Chosen by Voters

even if you send out a save the date about now gives people notice but 4-6 weeks is fine just be reminded that some people will take offensive my asking no kids

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