Can somebody please suggest what is the best time to prune so I get more flowers next summer.

The best time is NO time :-)) I don't mean to be facetious but hydrangeas that produce colored flowers (blue, purple, pink, white or mixed) in the mophead or lacecap form, bloom primarily on growth that was produced the previous growing season. It is my suggestion that these types of hydrangeas should be sited where they can be allowed to grow to size without the need for regular pruning. If you prune in late summer through spring, you risk removing most of the flowering potential. Deadheading or just removing spent flowerheads can be done whenever, provided you only remove down to the next set of buds below the flower. Removing dead growth stems can be done whenever you want - the suggestion of waiting until Memorial Day is that any healthy stems with new bud growth should be readily visible by then so it is easy to make the distinction. If your hydrangeas bud out earlier than that date, you can certainly remove the dead stems at that time.

But neither of these practices are pruning for size. If you DO need to prune the hydrangea due to size, etc., there are a couple of suggested methods. The first is rejuvenation pruning or the removal of upto 1/3 of the plant - typically the oldest stems - to the ground in spring. While this is an extended project, it does serve to reduce the overall size of the plant for a period of time while maintainting a decent bloom. It also serves to encourage new, vigorous growth from the root crown.

The second method is a one time reduction in the total shrub to the height you desire - i.e., cutting back a 5' tall shrub to a uniform 2'. You will sacrifice most, if not all, of the flowers but only for a single season.

You should also be aware that the latent flower buds produced by these types of hydrangeas can be damaged by winter cold or late spring frosts, which can also negatively impact flowering. Employing some sort of winter protection can avoid this.

There are a number of new cultivars of Hydrangea macrophylla on the market that have been bred to bloom on both old and new growth. These were developed specifically for cold weather gardeners to enjoy the flowers despite any winter damage mentioned above. Theoretically, these types can be pruned back as well and still flower, however again, you will get the best flower display if you can avoid annual pruning (and any winter damage).

Here is a link that might be useful: pruning Hydrangea macrophylla

Trim it after the first flush of flowers next summer. Since your hydrangea died down to the base last winter and still flowered in blue, I would guess you have the newer types that flower on both old and new wood, such as Endless Summer.

If it's already too big for you in one season's growth from the base, then maybe the wrong plant is planted for that spot. You could move the hydrangea now.

Pruning after flowering ensures you will still get all the flowers and by keeping long stems over winter, maybe you provide some winter protection to the inner parts of the plant as well. The new growth will then get a chance to flower a second time, since you have a reblooming type, butblooming may be sparse. The stems will have a chance to form buds for next year.
Prune off dead parts when new growth starts in spring and you can see what survived and what is dead.

@gardengal48
Many thnks for your prompt reply and the comprehensive information provided. The link is also quite infomative.
I definitely have the blue mophead variety. Altough larger than I want I think i'l just leave it alone and not prune at all. I don't want to miss the lack of flowers next summer.

A good number of older cultivars of macs show remontant tendencies - that's where the new "reblooming" varieties came from, afterall - so it's not entirely impossible that even hard pruning or winter damage will remove ALL flowers.

And I worry about using terms like 'first flush' with
regards to hydrangeas. They have markedly different bloom times depending on where in the country one is located. In more northern locations, they may not even start blooming until late June or July and continue on into August/September and don't exactly bloom in 'flushes' - what flower buds are there, are already there on the vast majority of cultivars. This is too late to prune hard and still expect good flowers the next year.

I stand by my suggestion that to maximize flower production, do NOT prune unless you absolutely have to.

@wannabeGardnr 7
Thank you for your advice I will consider it in future years.
In the mean time I will leave it alone and see what the summer brings. Although I will prune out any dead stalks that remain once I see the plant flowering.