last time i looked at giving blood, you have to book an appointment weeks in advance, thats not going to work for me 
		 
		
	 
This really confuses me.  I'm not being awkward, but why is this a problem?  I see it all the time.  This is not just a dig at you 
If you have a calendar, or a diary, if something has to be booked weeks in advance, you look in your diary/calendar/whatever and see if the time and date is free.
If it is, then you put that event in there.  It's booked, your time is reserved for it.
Then if someone else asks you to do something at that time and date, you can look in your calendar/diary/whatever and see that you're already booked for that time.
This literally happens so often.
"Hey guys, fancy a curry?  How about three weeks next Friday?"
"lol, I don't know what I'm doing tomorrow, never mind in three weeks time."
So you try the alternative....
"Hey guys, fancy a curry on Friday?
"lol, I've got something else arranged"
AAAARRRRRGGGGGGHHHHHHHH!
/end rant.