Dearfriend,
Thankyouforyourasking.Iwouldloveto,butIamreallyalittlebitbusythatday.Ihavetodosomethingimportant.IamverysorrythatIcannotgotothethere.I'mreallysorryaboutthat,Iwouldlikeverymuchtojoinyouanothertime.Ipresenttoyouthemostsincerebestwishes
Happybirthdaytoyou!Ihopeyouhaveagoodtime.
Yours,
Mary