משנכנס אדר מרבין בשמחה
No such Halacha
The Gemoro (Ta’anis 29a ) In the name of Rav
it was said, "the same way, when
the month of Av comes in, you reduce with Simcha,
so too when the month of Adar comes
in you add with Simcha".
The Rambam and Sh. Aruch bring the Halacha of
משנכנס אב ממעטין בשמחה
They do not** mention משנכנס אדר מרבין בשמחה
The obvious reason is, there is no such Halacha
to be Marbeh B’simcha when Adar comes in.
The Chasam Sofer (O.Ch 160) explains the reason.
"Since we don't Pasken like Rav that Aveilas starts on
Rosh Chodesh Av,(According to strict halacha it starts on
on Erev T.B.***), therefore we also don't Pasken
that Simcha starts on Rosh Chodesh Adar.
The Halacha in Sh. Aruch & Rambam of משנכנס אב ממעטין בשמחה
applies to only one thing, not to build a
Binyan Shel Simcha. All other types of refraining Simchas like
eating meat, drinking wine etc. start only on Erev T.B.
On this halacha we can't say The same as when Av comes
in you are not allowed to build Binyan Shel Simcha-- so too
when Adar comes in you may build a Binyan
Shel Simcha. Building a binyan Shel Simcha may be done all year round not only in Adar.
There is no such Halacha of Marbim Besimcha for Adar whereas there is a
Halacha in Rema to be Marbeh Ketzas Simcha on Lag Baomer.
It seems that a קצת שמחה mentioned in Shulchan Aruch is a greater שמחה
than a complete שמחה of משנכנס אדר that is not mentioned in Shulchan Aruch.
* According to the Shulchan Aruch & Rambam
** The Magen Avraham does mention it
***3 weeks, 9 days & Shavua Shechal Bo are Minhag