These instructions are for those using ENVI version 5.6.1.
Sentinel 2 data use very long folder and filenames that may cause problems in the Windows operating system. You should extract (unzip) the data into a folder near the top of the file structure. By this we mean U:\ rather than something like: U:\Project\Rasters\Sentinel\MyNewData\ImageDate. Once the data are extracted rename the new data folder to a shorter name. Sentinel 2 file name usually takes this fomat:
MMM_MSIXXX_YYYYMMDDHHMMSS_Nxxyy_ROOO_Txxxxx_<Product Discriminator>.SAFE
For example, a Sentinel data folder name might be:
S2A_MSIL1C_20211019T154251_N0301_R011_T18TXL_20211019T192902.SAFE
Which reflects the following infomration: an image obtained via Sentinel 2A (S2A), processed at Level L1C (MSIL1C), acquired on the 19th day of October of 2021 at 3:42:51 PM UTM (20211019T154251), processed with PDGS processing baseline 03.01 (N0301), Relative Orbit Number 011 (R011), Tile Number 18TXL (T18TXL), and its Product Discriminator is 20211019T192902. SAFE is the product format which stands for Standard Archive Format for Europe.
You can rename this file to something like: October19_2021.
To open the image, go the main ENVI menu and select File | Open As | Optical Sensors | European Space Agency | Sentinel 2. Navigate into the new folder and select the XML file. This, however, will be shorter filename such as:
MTD_MSIL2A.xml
ENVI should open the data into files based on spatial resolution. You can examine the data and save the file to ENVI format if you wish to use these data in the future. From the ENVI main menu select File | Save As | Save As… (ENVI, NITF, TIFF, DTED) and follow the instructions to save this file to a new folder structure for your project; it should not be placed in any of the original Sentinel folders. Consider including 10m or 20m as part of the filename to distinguish the resolution.
You can also open these data using the ESA Sentinel-2 Toolbox program SNAP. This is installed on the YCEO Lab systems. You can also download this software from the ESA site:
https://sentinel.esa.int/web/sentinel/toolboxes/sentinel-2
The ESA website has links to documentation and YouTube videos to instruct users. While you can view and manipulate these data quite easily in SNAP, it is a bit difficult to export these data into a format that can be used by ENVI. In order to export these data, they must all have a common spatial resolution. So if you want to use the four 10-meter bands you must resample the entire file to 10 meters. (this is a very large file!) You can then open this in ENVI and spectrally subset this to extract just the four bands of interest.