CREATE TABLE MONTHES (
    CODE    INTEGER NOT NULL,
    RSNAME  CHAR(3),
    RNAME   VARCHAR(8),
    LSNAME  CHAR(3),
    LNAME   VARCHAR(9)
);
ALTER TABLE MONTHES ADD CONSTRAINT MONTHES_PK PRIMARY KEY (CODE);
/*Заполняем 12 записей в таблице MONTHES 1 раз ручками с русскими и латинскими названиями*/

CREATE TABLE CALENDAR (
    CODEPER    INTEGER NOT NULL,
    PMONTH     INTEGER NOT NULL,
    PYEAR      INTEGER NOT NULL,
    NAME       VARCHAR(32),
    PERSTART   DATE NOT NULL,
    PEREND     DATE NOT NULL,
    DAY_COUNT  COMPUTED BY (EXTRACT(DAY FROM PerEnd - 1)),
);
ALTER TABLE CALENDAR ADD CONSTRAINT CALENDAR_PK PRIMARY KEY (CODEPER);
ALTER TABLE CALENDAR ADD CONSTRAINT CALENDAR_FKM FOREIGN KEY (PMONTH) REFERENCES MONTHES (CODE);
CREATE UNIQUE DESCENDING INDEX D_END ON CALENDAR (PEREND);
CREATE UNIQUE INDEX D_START ON CALENDAR (PERSTART);

create or alter procedure FORM_CALENDAR
as
declare variable PERSTART date;
declare variable PEREND date;
declare variable PMONTH integer;
declare variable PYEAR integer;
declare variable NMONTH integer;
declare variable NYEAR integer;
declare variable STRDATE varchar(10);
declare variable STRMONTH varchar(8);
declare variable PNAME varchar(32);
declare variable DATES date;
declare variable DATEE date;
declare variable ENOUGH integer;
Begin Enough=0;
  While (Enough=0) Do
  begin
    Select First 1 PerStart, PerEnd, PMonth, PYear From Calendar
     Order By PerStart Desc
     Into :PerStart, :PerEnd, :PMonth, :PYear;
    if (PerStart>'today'+730) then
     Enough=1;
    else
     begin
       if (PMonth=12) then
        begin PYear=PYear+1; PMonth=1; NMonth=2; NYear=PYear; end
       else
        begin PMonth=PMonth+1;
          If (PMonth=12) then
           begin NMonth=1; NYear=PYear+1; end
          else
           begin NMonth=PMonth+1; NYear=PYear; end
        end
       if (PMonth<10) then
        StrDate='0'||Cast(PMonth As Char(1));
       else
        StrDate=Cast(PMonth As Char(2));
       Select RName From Monthes Where Code=:PMonth
       Into :StrMonth;
       StrDate=StrDate||'.'||Cast(PYear As Char(4));
       PName=StrDate||' '||StrMonth;
       StrDate='01.'||StrDate;
       DateS=Cast(StrDate As TimeStamp);

       if (NMonth<10) then
        StrDate='0'||Cast(NMonth As Char(1));
       else
        StrDate=Cast(NMonth As Char(2));
       StrDate='01.'||StrDate||'.'||Cast(NYear As Char(4));
       DateE=Cast(StrDate As TimeStamp);

       Insert Into Calendar (Name, PerStart, PerEnd, PMonth, PYear)
       Values (:PName,:DateS,:DateE,:PMonth,:PYear);
     end
  end
end;
/*Вызов процедуры вструмляем в ночной крон и забываем о ней навсегда*/

/*Месячные, пардон, запросы будут выглядеть так*/
Select чо надо
From Calendar C
 Join Таблица_с_чо_надо T On T.Data>=C.PerStart And T.Data<C.PerEnd
Where C.PerStart<=Now And C.PerEnd>Now
/*+ условия по смыслу на таблицу с чо надо
   Now - частный случай, любая интересующая дата*/