📖 Update docs and issue templates with 2nd-Monday LTS release schedule (

#26256)

* Add how to determine if a release will become LTS

* Update release-schedule and lts-release docs with 2nd Monday schedule

* Explain what happens when Monday is a holiday

* Clarify copy in release tracking issue templates

* Tweak language around non-LTS releases

* Update LTS readme with notes about code freezes/US holidays

* /Monday/Tuesday/s

Co-Authored-By: Esther Kim <44627152+estherkim@users.noreply.github.com>

* Reword schedule details around release freezes

* Clearify language around release freezes

Co-authored-by: Esther Kim <44627152+estherkim@users.noreply.github.com>

.github/ISSUE_TEMPLATE/release-tracking-issue.md

@@ -36,9 +36,13 @@ Note: remove the backticks (``) from the link.
 - [ ] Release <RELEASE_NUMBER> promoted to Stable channel (<CL submit time>)
 
 <!--
-If this release will become the next LTS release, add/update the checkboxes above as needed e.g.
+On the second Monday of each month, the current stable version will be promoted to the LTS release channel. In other words: 
+
+- If this release is promoted to stable on the first Tuesday of a given month, it needs to be promoted to LTS on the second Monday of the same month.
+- If this release is promoted to stable on the second, third, fourth, or fifth Monday of a given month, it is not an LTS release candidate (but will be included in a later LTS release).
+
+Based on the above, if this release must be promoted to LTS, copy-paste the following checkbox into the list above.
 
-- [x] Release <RELEASE_NUMBER> will be the next LTS
 - [ ] Release <RELEASE_NUMBER> promoted to LTS (<CL submit time>)
 
 If you perform cherry picks, add/update the checkboxes above as needed e.g.

contributing/lts-release.md

@@ -1,6 +1,8 @@
 # Long-Term Stable (lts) Release Channel
 
-The **lts** release channel provides a previous **stable** build for one-month intervals. Approximately monthly, the current **stable** release is promoted to **lts**. This channel is not recommended for all AMP publishers. It is provided so that publishers who wish to perform a QA cycle on their website less frequently may do so by opting specific web pages into the **lts** channel.
+The **lts** release channel provides a previous **stable** build for one-month intervals. On the second Monday of each month, the current **stable** release is promoted to **lts**. This channel is not recommended for all AMP publishers. It is provided so that publishers who wish to perform a QA cycle on their website less frequently may do so by opting specific web pages into the **lts** channel.
+
+In the event that the second Monday of the month falls during a [release freeze](https://github.com/ampproject/amphtml/blob/master/contributing/release-schedule.md#release-freezes) (such as on one of the major US holidays), the promotion will be performed after the end of the release freeze.
 
 > Note: The **lts** release channel is available only to AMP Websites; it is not available to AMP Actions, AMP for Email, AMP Stories, or AMP for Ads.
 
@@ -32,4 +34,4 @@ The **lts** release channel provides a previous **stable** build for one-month i
 
 While it is a requirement that the same version is used for the runtime and extension scripts, requesting **lts** is valid AMP. It will get the same cache benefits as the stable release channel.
 
-**Important**: Publishers using the **lts** release channel should be careful when using newly introduced features. Because of the longer cycle, the **lts** release may be as much as six weeks behind the `HEAD` of [`ampproject/amphtml`](https://github.com/ampproject/amphtml). See the section on [determining if your change is in a release](https://github.com/ampproject/amphtml/blob/master/contributing/release-schedule.md#Determining-if-your-change-is-in-a-release) to validate if a change will be ready with your chosen release cycle.
+**Important**: Publishers using the **lts** release channel should be careful when using newly introduced features. Because of the longer cycle, the **lts** release may be as much as seven weeks behind the `HEAD` of [`ampproject/amphtml`](https://github.com/ampproject/amphtml). See the section on [determining if your change is in a release](https://github.com/ampproject/amphtml/blob/master/contributing/release-schedule.md#Determining-if-your-change-is-in-a-release) to validate if a change will be ready with your chosen release cycle.

contributing/release-schedule.md

@@ -27,7 +27,9 @@ The **stable** release channel is built with the _production_ configuration and
 
 ### Long-Term Stable (lts)
 
-The **lts** release channel provides a previous **stable** build for one-month intervals. Approximately monthly, the current **stable** release is promoted to **lts**. This channel is not recommended for all AMP publishers. It is provided so that publishers who wish to perform a QA cycle on their website less frequently may do so by opting specific web pages into the **lts** channel (see the [**lts** readme](https://github.com/ampproject/amphtml/blob/master/contributing/lts-release.md)).
+The **lts** release channel provides a previous **stable** build for one-month intervals. On the second Monday of each month, the current **stable** release is promoted to **lts**. This channel is not recommended for all AMP publishers. It is provided so that publishers who wish to perform a QA cycle on their website less frequently may do so by opting specific web pages into the **lts** channel (see the [**lts** readme](https://github.com/ampproject/amphtml/blob/master/contributing/lts-release.md)).
+
+In the event that the second Monday of the month falls on a holiday, the promotion will be performed after the end of the release freeze.
 
 Important: Publishers using the **lts** release channel should not use newly introduced features. Because of the longer cycle, the **lts** release may be as much as seven weeks behind the `HEAD` of [`ampproject/amphtml`](https://github.com/ampproject/amphtml). See the section on [determining if your change is in a release](#Determining-if-your-change-is-in-a-release) to validate if a change will be ready with your chosen release cycle.
 
@@ -101,6 +103,7 @@ If a one week release freeze is announced for Week N:
 - The previous week's release build remains in **experimental**/**beta** for an extra week, i.e. the release cut in Week N-1 is not pushed to **stable** in Week N as would normally be the case. Instead, it will be pushed to **stable** in Week N+1.
 - A new release build is _not_ made in the freeze week (Week N).
 - The normal schedule will resume in Week N+1, i.e. **experimental**/**beta** are cut in Week N+1 and promoted to **stable** in Week N+2.
+- If the **stable** release promoted during Week N-1 was originally scheduled to be promoted to **lts** during Week N, it will now be promoted to **lts** on the Monday of Week N+1.
 
 A release freeze may happen due to: