When getting an invitation to call, particularly the first time, what I like to know is:
The date. Weekday name and date number are helpful in case I misread my calendar. For example Thursday, January 5, 2012. All that redundancy make sure the caller and dancers arrive on the same day!
The place. A brief description of the facilty or venue is helpful (raised stage, dais, floor, size, features pertinent to the performance).
The times (plural). When the venue opens to the performers. When I should be there. When to start calling. Break time and length. When to stop the dance.
Who is providing sound equipment? Should I bring my sound system? I'll have my own mic available, but I can bring a compact sound system suitable for small halls.
Your program expectations. I call contras. I don't (yet) call weddings, dance parties, or family/community dances. Within the realm of contras it's helpful to know the mix of dancer experience expectations.
What terms do you prefer? I have no preference. In my experience organizers ask for either Robins/Larks or Ladies/Gents.
What is the expected pay being offered?