Tosefes Rosh Chodesh:[1]
There is a Biblical command to add to the time of the Shabbos and holidays in both its entrance and exit.[2] This means that one is to take in the Holiday prior to sunset and delay the leave of the Holiday past nightfall. It is disputed whether this Mitzvah applies also on Rosh Chodesh. Some Poskim[3] rule that this command does not apply on Rosh Chodesh, and there is hence no need to extend any of the Rosh Chodesh laws past their regular time of entrance and leave. However, from other Poskim[4] it is evident that the Mitzvah applies likewise on Rosh Chodesh, and one is hence to add to its Kedusha both by its entrance and exit.
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[1] Sichos Kodesh 1978 2/231 [printed in Shulchan Menachem 2/204]
[2] 261/4 regarding the entrance and exit of Shabbos; 293/1 regarding Motzei Shabbos; 608/1 regarding entrance and exit of Yom Kippur, Shabbos and all Holidays
[3] Opinion in Admur 188/17; Shelah brought in M”A 417/1
[4] Rebbe in Sichos Kodesh 1978 2/231 [printed in Shulchan Menachem 2/204] that according to those Poskim who hold that there is Tosefes Kedusha on Tishe Beav [See Beis Yosef 553] then certainly it applies on Rosh Chodesh.
How practically is one to fulfill the mitzvah of tosefes Rosh Chodesh?
On Shabbos we refrain from melacha from before the zman, the noshim light candles before shkia, and many people daven Maariv or make Kiddush early, such as in the summer months. But how is one to “add” to Rosh Chodesh, which in any event does not contain an issur melacha (for men), unlike Shabbos and Yom Tov?
Would this mean, for example, that one could not cut his nails one minute before shkia on erev Rosh Chodesh?