It's pronounced like the "ye" in "yesterday."
*Note: When "ě" follows the letter "m", it is pronounced as if there was a letter "n" between them. For example, the word "měkký" means "soft" and is pronounced "mněkký."
"Dobrý víkend" litterally means "good weekend". That wouldn't be logical. "Hezký víkend" is better, because it means "have a nice weekend", even though "have a good weekend" sounds natural in English. Czech is very different.