I see nobody has answered you yet, so I'll give it a try. This is a big topic! I'm no expert, but perhaps some others will chime in with their suggestions too. There's a whole lot more about this than I have covered here, but this will hopefully get you started. It will help to know where you are located and, to some extent, what breed you have. What kind of facilities do you have?
I don't know if heifers tend to calve earlier than mature cows or not. I do know that 2 weeks before or after a due date is possible and normal.
I hope you are not in a wickedly cold area. Calving in the midst of winter can be a challenge (but probably better than in July or August). Anyway, calving out heifers has some risks; they're as new to it as you are and don't always know what to do when a new baby arrives. Sometimes you have to assist a new mother; other times they do just fine on their own.
You'll want to keep a close eye on the heifers. Familiarize yourself with signs of labor. Watch to see if she isolates herself from the others. Is she holding her tail up at all? Sometimes they have mucous hanging from their vulva before calving (could be a day or a month before). Heifers' udders may/may not bag up before calving. If the teats are sproinging (my word) out to the side, she's fairly close. If she's getting up and down, up and down, she's probably in labor. This is not a complete list of imminent calving signs, so you'll need to do a little research and reading (suggestions below).
If you can, move her into a smaller enclosure with a shelter (again, depending on your climate) where you can keep an eye on her and won't have to traipse all over the countryside in the middle of the night searching for her. Make sure she has hay and water. Check on her frequently; if she's in active labor, I wouldn't leave her overnight without checking.
Have your vet on speed dial. Let him/her know beforehand that you have heifers due to calve and may be calling him. Have some colostrum supplement on hand (Life Line is a good one) -- better to have and not need than vice versa. Have a bottle of Tincture of Iodine 7% on hand to dip the calf's navel. Your vet can get you these things (or your feedstore).
Remember, often a cow or heifer will experience tremendous hormonal up's and down's. When a calf arrives, she may not want you coming over to inspect it. She'll toss her head up and down at you; there is no mistaking what she's saying. Do not place yourself in danger.
I have many websites bookmarked; here are a few that might be helpful to you:
http://www.noble.org/ag/Livestock/Minim ... index.html
http://www.cvmbs.colostate.edu/ilm/proi ... /table.htm
http://extension.missouri.edu/publicati ... px?P=G2007
http://www.angusbeefbulletin.com/Articl ... hecold.pdf
http://www.usask.ca/wcvm/herdmed/applie ... ernal.html
I would suggest you start googling calving websites and reading as much as you can. I would also recommend a couple of books by Heather Smith Thomas:
Essential Guide to Calving and
Getting Started with Beef and Dairy Cattle. Read, read, read!
Wishing you the best....this is exciting and scary all at the time same, but hopefully they'll all calve safely for you!
