Ah, so it's just the latest period.
I have to do something similar - a calendar year special category in a date dimension that has a financial year (may-april)
To accomplish this, I change the special category in the script that builds the cube according to the current date. For example, this month, the category is changed to 7 periods to give a January to July range. I also change the label to match the year (and remove those annual update headaches).
You'd need to consider what is required on the date of creation. For example, if created on the 20th July, should the 1-21 category be 1-21 June or just 1-20 July?
soi la, soi carré