Question: What is the phrase that is commonly used on the day which is observed on 25 April each year?

Context paragraphs:
Factor analysis: Factor analysis is a statistical method used to describe variability among observed, correlated variables in terms of a potentially lower number of unobserved variables called factors.  For example, it is possible that variations in six observed variables mainly reflect the variations in two unobserved (underlying) variables.  Factor analysis searches for such joint variations in response to unobserved latent variables.  The observed variables are modelled as linear combinations of the potential factors, plus "error" terms.  Factor analysis aims to find independent latent variables.  Followers of factor analytic methods believe that the information gained about the interdependencies between observed variables can be used later to reduce the set of variables in a dataset.  Factor analysis is commonly used in biology, psychometrics personality theories, marketing, product management, operations research and finance.  Users of factor analysis believe that it helps to deal with data sets where there are large numbers of observed variables that are thought to reflect a smaller number of underlying/latent variables.  It is one of the most commonly used inter-dependency techniques and is used when the relevant set of variables shows a systematic inter-dependence and the objective is to find out the latent factors that create a commonality.

Oh, with the verbing!: Oh, with the verbing!  is a phrase commonly used in TV shows and movies, first used by comedian Jerry Lewis.  The literal phrase "Oh, with the verbing!"  itself is normally used with a verb replacing "verbing," such as "screaming, stabbing, bleeding," hence the name of the phrase.  Similar lines related to the phrase are spoken as "The verbing, and the verbing" and "Again with the verbing" and other similar dialogue.

Lest we forget (phrase): The phrase "Lest we forget" is commonly used in war remembrance services and commemorative occasions in English speaking countries, in particular Remembrance Day and ANZAC Day.  Before the term was used in reference to soldiers and war, it was first used in a 1897 Christian poem written by Rudyard Kipling called "Recessional".  The phrase occurs 8 times; and is repeated at the end of the first 4 stanzas in order to add particular emphasis regarding the dangers of failing to remember.

Have a nice day: Have a nice day is a commonly spoken expression used to conclude a conversation (whether brief or extensive), or end a message by hoping the person to whom it is addressed experiences a pleasant day.  Since it is often uttered by service employees to customers at the end of a transaction, particularly in Israel and the United States, its repetitious and dutiful usage has resulted in the phrase developing, according to some journalists and scholars, especially outside of these two countries, a cultural connotation of impersonality, lack of interest, passive–aggressive behavior, or sarcasm.  The phrase is generally not used in Europe, as some find it artificial or even offensive.  Critics of the phrase characterize it as an imperative, obliging the person to have a nice day.  Other critics argue that it is a parting platitude that comes across as pretended.  While defenders of the phrase agree that "Have a nice day" can be used insincerely, they consider the phrase to be comforting, in that it improves interactions among people.  Others favor the phrase because it does not require a response.

Virtual population analysis: Virtual population analysis (VPA) is a cohort modeling technique commonly used in fisheries science for reconstructing historical fish numbers at age using information on death of individuals each year.  This death is usually partitioned into catch by fisheries and natural mortality.  VPA is virtual in the sense that the population size is not observed or measured directly but is inferred or back-calculated to have been a certain size in the past in order to support the observed fish catches and an assumed death rate owing to non-fishery related causes.

Divers hands: Divers hands (or more rarely dyvers hands), is an archaic phrase used to refer to a project that has been contributed to by many people. " Divers" is a word of Latin origin ("diversus") that is still commonly used in modern French language; it literally means "many and varied".  This usage of the word "Divers" can be found in the Bible and other older texts, but it is not commonly used in modern English.  The phrase is still used to refer to the authorship of plays, essay collections, and short story collections by multiple authors.

Set phrase: A set phrase or fixed phrase is a phrase whose parts are fixed in a certain order, even if the phrase could be changed without harming the literal meaning.  This is because a set phrase is a culturally accepted phrase.  A set phrase does not necessarily have any literal meaning in and of itself.  Set phrases may function as idioms (e.g. "red herring") or as words with a unique referent (e.g. "Red Sea").  There is no clear dividing line between a commonly used phrase and a set phrase.  It is also not easy to draw a clear distinction between set phrases and compound words.

Substitute teacher: A substitute teacher is a person who teaches a school class when the regular teacher is unavailable; e.g., because of illness, personal leave, or other reasons.  "Substitute teacher" (usually abbreviated as "sub") is the most commonly used phrase in the United States, Canada, India and Ireland, while supply teacher is the most commonly used term in Great Britain and the Canadian province of Ontario.  Common synonyms for substitute teacher include relief teacher or casual teacher (used in Australia and New Zealand) and "emergency teacher" (used in some of the United States).  Other terms, such as "guest teacher", are also used by some schools or districts.  Regional variants in terminology are common, such as the use of the term teacher on call (T.O.C.) in the Canadian province of British Columbia and occasional in Ontario.

All flesh is grass: All flesh is grass is a much-quoted phrase from the Old Testament, Isaiah 40:6 (Hebrew: כָּל־הַבָּשָׂ֣ר חָצִ֔יר‎ ‎ "kol habbasar chatsir").  In the New Testament the phrase reoccurs in the First Epistle of Peter (see ; Greek: πᾶσα σὰρξ ὡς χόρτος , "pasa sarx hōs chortos").  It was a commonly used epitaph, frequently found for example on old ledger stones and monuments in churches in 17th century England.  The phrase is interpreted to mean that human life is transitory.  It has been used in various works, including:

Anzac Day: Anzac Day is a national day of remembrance in Australia and New Zealand that broadly commemorates all Australians and New Zealanders "who served and died in all wars, conflicts, and peacekeeping operations" and "the contribution and suffering of all those who have served".  Observed on 25 April each year, Anzac Day was originally to honour the members of the Australian and New Zealand Army Corps (ANZAC) who fought at Gallipoli against the Ottoman Empire during World War I.

Supporting paragraphs: Lest we forget (phrase) Anzac Day
Answer: "Lest we forget"