Refineries will shut down various units or sections of the plant every 3 to 5 years. They perform planned inspections and maintenance for roughly a month. Prior to shutting down, all sorts of inventory is backed up. Unplanned shutdowns can stress the markets locally, until things are fixed, or until outside supplies are shifted in.
To get ready for summer gasoline, inventories are purged or moved around, and a few knobs and settings are adjusted. The biggest cost impact, In winter, is use of cheaper components, mainly butane. These are needed to keep gasoline vapor pressure high enough from cold fuel tanks.