Hi,
To accomplish this you will need to create set up your Watering program as you desire except you can ignore setting the days of the week and do not enable any start times. Just enter the run times for each station and make sure each station's Enabled check box is checked. I am going to show you a complete example below for your specific needs.
So I'll use Watering Program 4 for this example. See screen shot below. As you can see there are 6 stations with run times and their enabled check boxes checked. See Below.

- Watering Program.jpg (187.28 KiB) Viewed 28137 times
After configuring a single program for your desired valve run times you will then create a
Watering Scene using the
Insteon Programs 'Edit Scene' link. Simply create a new scene by entering a meaningful name and click the Create Scene Button. In this example we are naming it 'Watering Scene' Then Load this scene using the pull down selection and clicking the Load Scene button. The text field will be blank. Enter the following scene text:
WATERCYCLE,RUN,NONE,1;2;3;4;5;6;7;15;16;17;18;19;20;21,4
Click Save Changes.
This will create a scene that will run
Program 4 on the calender days shown in the list. Note that the days in the scene are
1-7 and
15-21 which will cover the first and third week of each month. The last number is the program number.
NOTE THAT THE DAYS ARE SEPARATED BY SEMICOLONS and not COMMAS. See below.

- Watering Scene.jpg (135.95 KiB) Viewed 28137 times
Now using the
Insteon Programs,
Scene Programs link, Select the scene name you created in the previous step using the pull down list, enter the desired start time and select
Sunday as the Day to Run the scene. Be sure to also check the
Enabled check box. See below.

- Watering Scene Program.jpg (139.56 KiB) Viewed 28137 times
Here is the resulting effect.
Every Sunday the scene will be run, but in the scene file it will verify that it is
only a Sunday that falls on the 1st through the 7th OR the 15th through the 21st and if so it will run the watering program, limiting watering to the 1st and 3rd Sunday of each month.
Don't forget you can also stipulate an
Execution time other than a specific time of day using the After Sunrise option vs a fixed time of day to run as indicated by the
red arrow. This was added as an example of another option you might want to implement vs the specific time of day. You would only pick one or the other of course. See the Edit Scenes page for details on all the other options and examples.
Please be sure to let us know how it worked out. Also if you need one-on-one help send support an email and we can get together with you on the phone if needed.
HydroFLASH Team