In general, the best time to prune is in winter or when deciduous trees have lost their leaves - December, January or February. However, dead, diseased or damaged wood can be removed/pruned out at any time.

As to technique, that can depend a lot on the species being pruned :-) Good pruning is more science than art and requires an understanding of plant morphology. Typically the primary suggestions are never top a tree, do not remove too much (no more than 1/4 to 1/3 at a time) and do not leave any stubs - prune branches or limbs back to the branch collar.

btw, trees seldom become overgrown. They will only grow to their potential or as much as conditions allow. The issue of being "overgrown" usually boils down to improper placement :-)

Elderberry is a large shrub really, since they will grow from the ground and sucker, they can be pruned like lilacs, which means they can be selectively cut to the ground when growth is recommencing or when most active, for us it would be very late winter or early spring. Oak trees can have their canopies thinned out and raised, but never should you ever top them or head them off, ideally in this climate, they’d be pruned while dormant, in your climate, I don’t know. It is never a good idea to significantly reduce the canopy before extreme drought, you should always avoid compounding stress upon stress. If the tree no longer fits in your space, it can be the kinder thing to remove it entirely.

We have California Live Oaks all over our neighborhood. They're evergreen and we usually have ours trimmed sometime between July and October. We are actually having them trimmed next week. We also have Crape Myrtles. It's been awhile but I'm pretty sure we've had them trimmed late winter to early spring.

Acorn, oak tree is very very overgrown and lop sided.It is next to the house, blocking sun from useful vegetable beds in my sideyard. Really would like to get rid of it, but feel guilty about cutting down a tree.
Hence extreme pruning is what I am thinking.

Just call a tree company tomorrow and ask them. You risk losing blossoms if you do it at the wrong time.

Ruchi, is it actually called an Acorn Oak? I think the correct name of our trees is Coast Live Oak.

https://ucanr.edu/sites/oak_range/Californias_Rangeland_Oak_Species/Coast_Live_Oak/

Thanks Chloebud, for the botanical reference. Are the acorns used for anything useful? Other than decorations.

A lot of different oaks can grow in California :-) Until we can ID it accurately, doesn't make much sense to assume it is a native species. ALL oaks produce acorns so that's not much of an idenitifier........

I've actually only used them for decor in the Fall (nice clean ones!). I put them in glass hurricanes as a "nest" for candles or mini pumpkins. They can look nice just on their own in a dish. I can tell you our grandson and the squirrels love to gather them up. Are you in SoCal? Just wondering if you also have a California Live Oak...?

Look at the size of the tree and a closeup for ID

Ruchi, it's hard to tell from your photos. Your tree looks much more dense than ours do. You can see the branches through them, but we have them trimmed regularly. Our oaks are really old and tall. This is a close up from online of our leaves. As you can see, the edges are "prickly" and can hurt.

Here's a photo of one of our neighbor's trees. Sorry I can't make it bigger but maybe you can see what I mean regarding seeing the branches.

ruchi,,,You may want to get a proper ID. I don't know where you live (Im in SoCal) and it's illegal to cut all native oak trees. I'm sure you can prune them. Call in an arborist. Your tree is so big, this isn't something you'd want to tackle yourself anyway.

Is it a Red Oak?

In my area, only certain companies with trained personnel are licensed and permitted to prune oak trees. Check with city hall before you get hit with a multi thousand dollar fine. Don't believe reassurances by neighbors, tree pruners, real estate agents from adjacent cities, gardeners or anyone not official- check the regulations at city hall.

Yes, oaks are protected here. Easy to check with an arborist or tree service, Not sure but I think even trimming might require a permit in some locations. Long story but years ago we had to have a couple healthy oaks removed due to problems with a neighbor. At the time, the tree guys confirmed the oaks were protected, but explained they could get around that by saying the trees were "diseased." It was awful...I literally cried. Now we live in an area known for all the beautiful old oaks and no one wants them gone.

Good advice