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My sister is getting engaged tonight please God. Is there any rabbinical ruling that would allow me to shave for the occasion?
The majority of poskim do not make a distinction between shaving and taking a haircut, and therefore the custom to refrain from haircuts will apply to shaving. See, for instance, Iggros Moshe, Orach Chaim 4:102, and others. Although some authorities are more lenient concerning shaving than a haircut (on account of their different natures — a haircut is something “special,” whereas many shave daily), the general custom is to refrain from shaving.
An engagement is not a Yom Tov or an occasion such as a Bris that permits shaving, and therefore there is no special permission to shave.
However, if remaining unshaven will cause embarrassment, there is room to permit shaving. Because the common custom is to remain unshaven during the sefira period, the ‘unshaven look’ is not usually embarrassing for this time, and therefore this will not apply.
A possible option is to keep the “second half” of the Sefira. This can be done if required, and you can therefore shave for the occasion and keep the Sefira from Rosh Chodesh and on (see Iggros Moshe, Orach Chaim 1:159).