2002 mercury sable. After sitting day or 2 doesn't start. Cranks. Will start right up when jumped. What's the issue?

WEAK or CHEAP battery or your charge system is BREAKING DOWN! AT THIS Age it is REASONABLE to say replace BOTH ALTERNATOR and the BATTERY! DO NOT use an AUTO ZONE BATTERY! Get a JUNK YARD battery from a RECENT WRECK, ALTERNATOR <TOO! GET it running AND GO TO a DIY car WASH WITH A CAN OF gunk original orange engine cleaner AND wash the battery AND alternator DOWN! clean the cables! iT MAY just BE corrosion oN THE BATTERY POSTS!  When the car SITS a while, it is SIL working to keep the CLOCK working and the RADIO STATIONS SETTING working too! CLEAN THAT BATTERY POST! ONLY get a battery with REAL CELL CAPS ON it! MAINENANCE FREE batteries are JUST another way of saying CHEAPER to make! 

Car battery on average the $100 bat. lasts about 6 years, so car is a 2 add 18 years gives you 20 and you are 1 month from 20.  Batteries last from 5 to 7 years(so it is an approximation) so the numbering fits.  You need a battery which is about $100. Do it soon because it is straining the alternator and the starter (so you don't want a "3 Amigos" situation if you burn them out(starter costs$400, alternator$400 AND YOU STILL NEED THE BATTERY. $100   They are all related and tied together as ELECTRICAL SYSTEM.  The car is not worth $1K when you got to also replace them because you cheaped out on the battery replacement (doing it quick)  Now that you know...."DO IT"  Just to save some money.  A running functioning "beater is worth $2K" so even considering to replace it with another car costs you MORE than just replacing the battery.  Most times the new beater you bought NEEDS A BATTERY.

. Cars do not know a day has passed....so it never AGES.(except for the Battery and the rubber hoses and belts and tires which are Time sensitive...rubber is good for 7-10 years then you start checking them.  Cars just age because you are FAMILIAR with them.(the fact is YOU aged).

45+years mechanic.

Replace the battery and the battery terminals.  Problem does not go away (wont start after sitting day or 2), see a mechanic to check for a battery drain.

Probably a weak battery. Or the car is dying of old age.

No idea but from past experience that usually points to a weak battery .. or possibly a weak coil..