You could compare the time your cat goes through then with a young girl's first menstruation. Therefore a midnight meeting with the neighbors' tom can bear adverse consequences.
Just a little Sex-Ed
Broadly speaking being "in heat" or "calling" is the time(s) of the year when the cat is able to become pregnant. The follicles (eggs) come to maturity in the cat's ovaries and contingent upon that as well as the day's length and the season hormones are released that make the cat change her behavior as well as her scent in a manner that will attract potent male cats.
Cats, contrary to us human-beings, can never get pregnant from the first mating. That is like that because ovulation can only be initiated by the pain the cat feels when she is penetrated by the tom's penis (which caries lots of small "barbed hooks" on it).
This is also the reason why cats in professional breeding have to "take the tom" several times.
In one kindle of kittens there can be up to three different fathers!
The In-Heat-Check-List
If your cat is in heat can it be checked best by watching her closely. The following check list will help you look out for the right signs.
1. Completely Mad
Cat owners who experienced their cat being in heat describe her during that time pretty often as "completely mad" and irrational which means that the usual character of the cat changes until almost nothing is left of it anymore. But don't be afraid - as soon as the heat is over your cat will cool down to her old shape again. ;-)
2. Talkative
Even calm and silent cats become talkative during this period and seem to be meowing all the time. Do you also feel slightly reminded of a teenage girl's behavior so far?
3. Clingy Kitty
Calling cats seek attention and love not only among other cats in the neighborhood but also from you as care giver. She will want to be petted and noticed between 22 and 24 hours a day but it also means she trusts and loves you (as much as any other tom near her...). ;-)
4. Tail up High!
Especially while petting her you will notice that as soon as you come close to her lower back her hindquarters will lift up and press firmly against your hand. Her tail will be upheaved and she will probably purr manically.
She does that to make mating easier for the tom.
5. Roller-Coaster
The best known sign for a cat being in heat is her constant rolling and lolling on the floor as well as their rubbing on all reachable furniture around. They also seem to be never sleeping - just like you (during her calling).
Therefore: If one or more of your cats shows signs of being in heat we advice you to call your vet and let her spay or castrate.
Picture: CC 2.0 by Zemlinki!