Under the Texas Standard Possession Order, April 1st of each year is the deadline for the non-primary parent to designate his or her 30 day extended summer possession period. The standard possession order states that April 1st is the deadline for the non-primary parent to give written notice of the extended possession period to the primary parent.
Remember, the non-primary parent still has the same 1st, 3rd, and 5th weekends as during the school year. The extended summer possession time is additional, and must be exercised in two periods. Each possession period must be at least seven consecutive days long. An extended possession period must end at least seven days prior to the child resuming school, and cannot begin before the child's school has ended for summer vacation.
As always, refer to the court's final decree/order if there's any confusion.