Short answer: Your deductible and out-of-pocket maximum reset at the start of each plan year, which is often January 1 but can be any 12-month period the employer chooses. Once the new plan year begins, your accumulators go back to zero.
Deductibles and out-of-pocket maximums reset on the first day of the plan year. Many plans run on a calendar year (resetting January 1), but an employer can set a plan year that starts in any month, for example July 1, and then the reset happens then. This matters for timing care: spending late in one plan year doesn’t carry over once the new year starts. If you switch plans or carriers mid-year, accumulators usually reset at that point too. Check your plan documents or SBC for your exact plan-year start date.