1. Major releases
2. Patch releases
3. Daily releases
Patch Releases and Daily Releases are used to deliver scheduled and ad hoc application fixes and are typically seamless to customers. Whenever possible, patches and daily releases are deployed during off-peak hours and without downtime. Patch releases are scheduled weekly and are usually deployed to instances on Tuesday, Wednesday or Thursday, with releases to Asia-Pacific instances the following day. Daily releases are conducted on an as-needed basis and can occur any day of the week.
Salesforce major release maintenance occurs three times per year. The instance will be unavailable for up to five minutes during the release window. The instance will be in read only mode.
BEFORE A SALESFORCE MAJOR RELEASE
1. Go to https://releasenotes.docs.salesforce.com and select the Release to review the Release Preview for new features.
Note:
The link usually shows the current release. Select the upcoming release to review.
2. Check Critical Updates which may affect your infrastructure. Example: Salesforce disabled TLS 1.0.
3. Sign Up for Pre-Release program to experience the release.
Sample URL:
https://www.salesforce.com/form/signup/prerelease-winter19/
Replace winter19 with the release you want to review.
4. Test in Sandbox Preview:
With every upcoming release, Salesforce provides a window during which you can refresh your sandbox and the new sandbox copy will be on the next release. This is a great way to test out your existing customizations against release features. Remember that creating a sandbox preview will require a new sandbox refresh.
Testing your existing customizations in a sandbox or a pre-release org ensures that your users don’t experience any interruptions post-release. One effective strategy is to build a test plan that covers all your customizations, and then ask your super users to help you test. You can use the plan for future releases, too.
AFTER A SALESFORCE MAJOR RELEASE
1. Smoke Test.
2. Train users on new features.
3. Create your release plan with your stakeholders and communicate it to your users.
Check this for more information from Trailhead module – https://trailhead.salesforce.com/content/learn/modules/sf_releases?trail_id=sf_release_prep