Ask anyone what Shavuos reminds them of, and they’ll likely respond eating cheesecake. This minhag seems to have been first mentioned by the Rema (OC 494:3) who explains that the extra dairy dish commemorates Shavuos’s unique Korban, the Shtei Halechem. Sefer Metaamei Moshe lists 149 different reasons for this minhag!
Maharam Mirottenburg (615) related that after eating hard cheese, he could
taste it hours later. The Rema (YD 89:2) paskens
that it is appropriate to wait after eating hard cheese before eating meat,
just as one must between meat and milk. R’ Moshe Feinstein (YD 2:26) points out
that this is only a stringency. Interestingly, the Maharshal (see Shach
YD 89:17) dismisses this custom as heresy as the Gemara (Chullin 105a) writes
that there is no need to wait between cheese and meat! The Gra disagrees,
saying that it is similar to adopting other personal stringencies. Nonetheless, this has become the accepted practice for Ashkenazim
(See Chochmas Adam 40:13). Sefardim
have various lenient opinions to rely on (See Yabea Omer YD 6:7).
what constitutes ‘hard cheese’ is a matter of much debate. Fatty, greasy
cheese and cheese that has developed holes would both qualify as ‘hard’ (See
Taz and Aruch HaShulchan YD 89:11). The Shach (YD
89:15) writes that cheese that has aged for 6 months, such as ‘swiss cheese’
(Taz 89:4) generally qualifies as hard cheese. While
some measure the 6 months from the time of production, R’ Yisroel Belsky maintains that
the cheese only matures while in the factory, before it is packaged. Mild
cheddar is typically a couple of months old, though mature cheddar is often
older than 6 months.
R’ Aharon Kotler held (Ohr Yisroel 6:p89) that only cheese which needs a sharp grater to cut up is considered ‘hard.’ Following this, R’ Moshe Heinemann classes parmesan cheese as one of the few ‘hard cheeses’. R’ Moshe Feinstein (Mishneh Halachos 16:9) and the Chazon Ish (Maaseh Ish 5:p22) likewise, held rather leniently.
The Yad Yehuda (89:26) writes that if hard cheese is used in baking, it loses its ‘hard cheese’ status when it melts. R’ Yosef Shalom Elyashiv (Kovetz Teshuvos 1:582) disagrees, however, as melting the cheese doesn’t change its taste.
While eating dairy foods is certainly an important minhag, the Darchei Teshuva (89:19) challenges it, saying that there is a more important mitzva to eat meat as part of Simchas Yom Tov and this mitzva applies during every meal. R’ Moshe Feinstein, however, notes (OC 3:68) that we eat meat to remember the Korban Shelamim which was only brought once a day. Thus, one could have both dairy meals to fulfill the Shavuos custom and meat meals to properly fulfil Simchas Yom Tov.