Usage of reschedule

[Gerald]

dear friend,

we are reschedule the meeting on 21 Dec.Tell us if you are free on that date.

[Lance]

Dear friend,
We wish to reschedule the meeting for 21 Dec. Tell us if you are free to attend on that date.

Writing "wish to" makes it clear that the meeting date is not firm yet. If the meeting date were firm, you should write, "We have rescheduled the meeting for 21 Dec. Tell us if you will attend."