Einstein died in 1955. Redshift observations started in 1912 but weren’t clear on what was happening or why, Friedmann et al’s calculations in 1922 probably raised the question in a serious context, and Hubble’s law was published in 1929. He knew about it, at least from the early '20s. Someone who read his letters or biography could probably narrow it down further.
Measuring red shift while observing stuff we see seems to get farther sway by the minute.
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Did Albert Einstein know about red shift?
Einstein died in 1955. Redshift observations started in 1912 but weren’t clear on what was happening or why, Friedmann et al’s calculations in 1922 probably raised the question in a serious context, and Hubble’s law was published in 1929. He knew about it, at least from the early '20s. Someone who read his letters or biography could probably narrow it down further.
https://wikipedia.org/wiki/Hubble's_law