Effective with the April administration of the LSAT-Flex, test-takers will be required to schedule their testing day and time by 11:59 p.m. ET of the Thursday prior to the exam. For example, for the April administration of the LSAT-Flex, which begins on Saturday, April 10, scheduling will close at 11:59 p.m. ET on Thursday, April 8. After that deadline passes, test takers could opt to request a date change for a future administration, but will not be able to schedule for the current exam.
Check out the LSAC website for more information.