Not only on Friday, I'd say the executors are killed when more jobs are started and waiting for executors. Usually, we see something like the following when the executors are killed (not sure if that is the cause, or consequence):
Creating placeholder flownodes because failed loading originals.
java.io.IOException: Tried to load head FlowNodes for execution Owner[continuous-integration/3.x/39:continuous-integration/3.x #39 (closed)] but FlowNode was not found in storage for head id:FlowNodeId 1:30
at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.initializeStorage(CpsFlowExecution.java:679)
at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.onLoad(CpsFlowExecution.java:716)
at org.jenkinsci.plugins.workflow.job.WorkflowRun.getExecution(WorkflowRun.java:701)
at org.jenkinsci.plugins.workflow.job.WorkflowRun.onLoad(WorkflowRun.java:560)
at hudson.model.RunMap.retrieve(RunMap.java:233)
at hudson.model.RunMap.retrieve(RunMap.java:61)
at jenkins.model.lazy.AbstractLazyLoadRunMap.load(AbstractLazyLoadRunMap.java:650)
at jenkins.model.lazy.AbstractLazyLoadRunMap.load(AbstractLazyLoadRunMap.java:632)
at jenkins.model.lazy.AbstractLazyLoadRunMap.getByNumber(AbstractLazyLoadRunMap.java:530)
at hudson.model.RunMap.getById(RunMap.java:213)
at org.jenkinsci.plugins.workflow.job.WorkflowRun$Owner.run(WorkflowRun.java:949)
at org.jenkinsci.plugins.workflow.job.WorkflowRun$Owner.get(WorkflowRun.java:961)
at org.jenkinsci.plugins.workflow.cps.CpsStepContext.getExecution(CpsStepContext.java:218)
at org.jenkinsci.plugins.workflow.cps.CpsStepContext.getThreadGroupSynchronously(CpsStepContext.java:243)
at org.jenkinsci.plugins.workflow.cps.CpsStepContext.access$000(CpsStepContext.java:98)
at org.jenkinsci.plugins.workflow.cps.CpsStepContext$1.call(CpsStepContext.java:264)
at org.jenkinsci.plugins.workflow.cps.CpsStepContext$1.call(CpsStepContext.java:262)
at jenkins.util.ContextResettingExecutorService$2.call(ContextResettingExecutorService.java:46)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
at java.base/java.lang.Thread.run(Thread.java:840)