water as needed and apply some calicum to help prevent end rot but beaware that temps also play a part here......

Most vegetables need from 1 to 1.5 inches of rain a week to grow well. If it doesn't rain, water slowly until the soil is moist at least 6 inches deep. Cooler weather means the soil moisture lasts longer and watering may not be needed as often. Hot weather later in the summer means you will need to monitor the moisture level more closely.

If you use a sprinkler, watering in the morning is preferred since it gives time for the moisture to dry off the leaves more quickly. Watering late in the day means the leaves will be damp for a longer period of time, which gives disease organisms a longer time to invade the tissues.

Keeping the soil moisture level fairly even helps prevent blossom end rot (BER) but it won't prevent it entirely. Plants can't take up the nutrients from dry soil, but as the previous poster mentioned, temperature plays a role also. A soil test will tell you if you need to add calcium. Usually the first tomatoes of the season are prone to BER but the situation improves when soil temperatures even out a bit later in the season. There are a number of sprays available that claim to prevent BER. I haven't seen any research saying whether they work or not.

Good luck with your garden.
Sandy