When and how is the right time to wean my child of the pacifier?

First ask yourself, why do you want to wean your child from his/her pacifier? If it's interfering with your child's ability to interact, talk, or play with others then yes I would agree with you it may be a good idea to phase it out. But if he/she is using it only to fall asleep or to soothe himself/herself when his/she's upset, then in my opinion he is using it to a good end. 

Like many parents, you may be concerned about the impact of pacifier use on your child's teeth, but pediatric orthodontists report that regular use of a pacifier before age 4 doesn't cause dental problems or misaligned teeth.

At preschool your son/daughter sees that the other kids don't use a pacifier so that more than likely why he/she has given it up on his own there.

Here are some helpful steps to wean your child from the pacifier:

Step 1 - Help him/her find other ways to calm himself. Many children use a security blanket or toy for comfort. If your child doesn't have this already, you can help him develop one by choosing a blanket, stuffed animal. Have it sit on your lap or your child's as you read together, and set it next to him as he/she eats and when you cuddle before bedtime.

Step 2 - Offer your child the opportunity to trade in the pacifier for a big-boy toy. If he doesn’t want too just yet don’t worry, it was worth a try.

Step 3 - Dip the pacifier in something that doesn't taste so great, like pickle juice, so your son/daughter will look forward to it less.

Step 4 - Taking the pacifier away gradually is the best way to go. Don't get into a power struggle over it or your child may hold onto it more fiercely. When your child wants the pacifier during the day, let him/her have it for a minute or two and then offer other ways to help him calm down, a hug from you, snuggling with his blanket or teddy, or reading a book together.

Step 5 - Reduce the time you let him/her have it until he doesn't use it at all.  

Step 6 - Taking the pacifier away at bedtime will probably be more difficult. Perhaps let him/her use it during reading time and while you're rocking or singing but not when he goes to sleep. Then gradually lessen the time he/she uses it.  It’s a long process and you need to be patient and calm.

Step 7 – When your child is ready to give up his pacifier altogether, it’s a great idea to have a pacifier going-away party, as special rituals can ease difficult transitions. Gather them up, put them in a special place (such as a box you've decorated together), sing some good-bye songs, include all your children and then put the box away.