Yevamos 43b - Aveilus Chadasha and Yeshana

The gemara concludes that a Nesuah cannot even do Eirusin during aveilus chadasha, but during aveilus yeshana it is permitted. But masah u'matan and laundering are the opposite, they are permitted during aveilus chadasha (during shloshim), but assur by yeshana (as a side point: the gemara seems to understand that shloshim of aveilus chadasha is equal to shavua shechal bo of yeshana).

2 separate points:

1. Tosafos is bothered about this distinction. Tosafos seems to explain that the 2 contrasts of the gemara of 1. yeshana vs. chadasha, 2. rabim vs. yachid, are coming to explain these distinctions. Regarding eirusin, we are more machmir by an aveilus chadasha since it is more chamur, rather than an old aveilus which we don't have to be so strict about. However, regarding masah u'matan and laundering, we are machmir by a public aveilus from doing public activities bec. it looks as if you are not concerned about the aveilus, but by a private aveilus where it is not obvious to others that you are in a state of aveilus we are not machmir about public activities since people don't realize that you are an avel. Therefore, the distinction between chadasha and yeshana explains eirusin, and the distinction between rabim and yachid explains masah u'matan and laundering.

2. Tosafos has an approach that the mitzvah of pru u'rvu would override 12 months for a father and mother but not shloshim (Rabbeinu Tam then goes further to suggest that it can even override shloshim). Similarly at the end of Tosafos there is a tzad that seudas mitzvah will override 12 months of a father and mother but not shloshim. If we assume that 12 months is a chiyuv of aveilus for a father and mother similar to sheloshim of other relatives, it is not clear why we should make a distinction. This distinction seens to be a support for R' Moshe that the aveilus of 12 months is not a din in aveilus (see also chochmas adam 166:2), rather a mitzvah of kibud av, therefore, a mitzvah cannot push off aveilus, but it can override the mitzvah of kibud av.