Ten months in advance

Choose a committee and elect leadership.  Hold first meeting and plan theme, reunion name, who to invite, date and begin searching for a location. 
The three sisters visited the hotel together and negotiated the terms of the contract. 
My own family pretty much took a week of vacation and put in for it early.  We wanted to make sure we could enjoy the early guests and finish wrapping up any loose details if necessary. 

Nine months in advance

Consider creating a website such as a free weebly.com one that the whole family can access for reunion information and forms. 
My sister and her husband created a website that included pictures of the t-shirt, any reunion information and had a blog.  As people decided to attend, they logged their information on the contact form and we had a running total of guests.  Everyone knew who was attending and how they were related.
The contact form was set so that if anyone left a comment, all committee members received an email. 
Finalize the hotel contract making sure you do not have to meet a catering minimum, and clearly know what is expected of the reunion committee and of the hotel. See if the hotel can provide brochures for your mailing next month. 
Begin designing the t-shirt and having it approved by the committee. 
Begin confirming the addresses you will need for the mailing next month.  Make sure you have everyone's address and that each address is correct. 

Eight months in advance

The first mailing should be sent with information regarding location, cost of hotel and hotel contact information to make reservations, what to expect in the months ahead that may cost (t-shirts), who is invited and the web address. 
The committee sent mailings to only siblings but the cousins sent one mailing about four months prior to all of the cousins. 
Make sure you use a reunion return address on the envelope and re-address any return mail you might receive.  It is important all guests have plenty of time to plan a vacation, budget for the reunion, etc. 
Let guests know who is on the committee and how to contact someone for questions they may have.

Seven months in advance

Plan the menu, discuss any purchases that will need to be made for the event.  Remember that by keeping the menu simple, you will more likely be able to enjoy your time with the guests.
Create a budget of projected expenses and see how your plans need to be adjusted.  Use this budget to determine how you might take advantage of sales in the coming months.
We were able to look at the complete list of items needed and decide who might already have things on hand to donate to the reunion.  The popcorn was budgeted but one sister had purchased a large amount to help out our nephew.  She donated more than enough popcorn. 
Another sister had the popcorn containers to dress up the gift baskets. 

Six months in advance

If you have decided to make any gifts or need to locate any items, this is the time to get the footwork done. 
By being organized, we were able to take advantage of finding our western decorations at Hobby Lobby for 50% off.
I had plenty of time to peruse thrift stores and Dollar Tree to find inexpensive theme items.  I found a fringed saloon door that we planned to purchase last minute for $10 on clearance for about $2 at Hobby Lobby several months before the reunion.
I found a great western metal sculpture several months before the reunion with the original price tag of $70 and now at a garage sale for $5 along with two western frames at the same sale for $.50 each that we used to display the pictures of two sisters who had passed away.
One of the touching memories at the reunion was this "memorial" table.  Priceless!!
We found cowboy hats for $1 at Dollar Tree and purchased 23 of them.  I never found them again so if we had waited to purchase, the price would have been much higher somewhere else. 

Five months in advance

If you know how you want to decorate the room try it for the committee.  We had a get together and tried out some of the decorations. 
The committee visited Constructive Playthings and purchased our cowboy hats and hat bands, priced some decorations and bandanas and just had fun.

Four months in advance

Second mailing should go out with a stamped, return addressed envelope if you need t-shirt forms, rooming list or airport itineraries returned. Make sure they know who to return each form to. 
We had all forms sent to one location and then passed them as necessary.
This letter again mentions the web address, the location and contact information, the rate, the amenities, the details of the t-shirt size, cost and deadline.  What they need to bring, swimsuit, sunscreen and includes a schedule of events.

Three months in advance

Make sure all t-shirt orders are being received to meet your minimum quantity.  Have all monies paid up front.  Order your t-shirts no later than two months in advance.

Two months in advance

Plan your reunion schedule/program.  Even if it is a one page flyer, communicate to everyone when the meals take place, when to meet for the family picture and any instructions for what to wear (t-shirt), etc.
If you ordered t-shirts, they should be arriving.  Verify the order and separate by who placed each order.
Make sure you have all items ready to place in the basket around one month prior to the event. 
We had the cake ordered two months prior as we wanted a custom cake and not one from a grocery store. 
Everyone should clearly know what responsibilities they have for food, decorations, transporting guests, etc. 

One month in advance

If you chose a hotel for the reunion, send the hotel a diagram of how you want them to arrange the meeting space.
Rooming list should be finalized and sent to the hotel no later than two weeks in advance. 
We gathered one month prior and labeled the water bottles, made sure all t-shirts were sorted by  the name of the person who placed the order, and we filled all of our baskets.  We placed the name tags in each basket so we could tell later who the basket belonged to. 

Two weeks in advance

Your rooming list and diagram should be taken care of by now.  You should be feeling pretty good about what you have accomplished. 
Be sure you have made plans to pick up guests from the airport if they are not renting a car.  If guests are arriving early, consider if you will be able to entertain them.  Try to keep yourself free to enjoy the company.
Are guests staying for a few days and do they need any transportation to the airport or might you invite them for dinner one night during their stay.
Is there any food that can be made ahead and frozen?  Are there any candies you are making or cookies that can be made ahead. 

One week in advance

The cousins filled cello Bandana bags from Hobby Lobby with a variety of 5 cookies each.  Each cousin made enough cookies for everyone and then they met two days prior to fill the bags.
We had a cake made and the day prior I picked up the cake, and flowers for the reunion. 
My daughter made the sign for the easel at the hotel the last week using her Cricut. 

OH NO! IT'S TODAY!!!!

We had such fun decorating while the early arrivals knocked on the locked door saying "Open, Open, Open".
We could have enlisted their help.  Heaven knows they were willing but again we wanted to honor them by having their first impression be of a fully decorated meeting room.  It was well worth the effort. 
It was such fun to open the doors and have everyone enter at one time and hear their sheer joy at what awaited them. 
Relax.  Know that by now all the relatives are so grateful for all the months of planning and now that they are in town, they want to help.  Use the time to visit with them while they work along with you.  You do not have to do it all alone.