You have probably already noticed that the order of loading the packages makes a difference, i.e. the package that gets loaded last will mask the functions in packages loaded earlier.
To specify the package that you want to use, the syntax is:
chron::is.weekend()
tseries::is.weekend()
In other words, use packagename::functionname()
In addition, if you know that you will always want to use the function in chron, you can define your own function as follows:
is.weekend <- chron::is.weekend #EDIT