Sometimes straight away, but more often around the RSVP date. Often I try to talk to the mum directly after school if I see her, so it is just whenever that happens.
As far as I am concerned if you need or want people to reply before the RSVP date, then make the date earlier. I always thought it was a reply by, not reply a week before this date.
Also don't always blame the parent, only last month I was handed an invite at 7pm Saturday night for a party on the Sunday
Bookmarks